nasm — the netwide assembler · 2002-09-03 · 7.5 interfacing to borland pascal programs . . . ....

NASM — The Netwide Assemblerversion 0.98.34

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Contents

Chapter 1: Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

1.1 What Is NASM?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

1.1.1 Why Yet Another Assembler?. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

1.1.2 Licence Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

1.2 Contact Information. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20

1.3 Installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

1.3.1 Installing NASM under MS−DOS or Windows. . . . . . . . . . . . . . . . . . . . . .21

1.3.2 Installing NASM under Unix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

Chapter 2: Running NASM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

2.1 NASM Command−Line Syntax. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23

2.1.1 The −o Option: Specifying the Output File Name. . . . . . . . . . . . . . . . . . . . .23

2.1.2 The −f Option: Specifying the Output File Format. . . . . . . . . . . . . . . . . . . .24

2.1.3 The −l Option: Generating a Listing File. . . . . . . . . . . . . . . . . . . . . . . . . .24

2.1.4 The −M Option: Generate Makefile Dependencies.. . . . . . . . . . . . . . . . . . . . .24

2.1.5 The −F Option: Selecting a Debugging Format. . . . . . . . . . . . . . . . . . . . . . .24

2.1.6 The −g Option: Enabling Debug Information.. . . . . . . . . . . . . . . . . . . . . . .24

2.1.7 The −E Option: Send Errors to a File. . . . . . . . . . . . . . . . . . . . . . . . . . . .24

2.1.8 The −s Option: Send Errors to stdout . . . . . . . . . . . . . . . . . . . . . . . . . .25

2.1.9 The −i Option: Include File Search Directories. . . . . . . . . . . . . . . . . . . . . .25

2.1.10 The −p Option: Pre−Include a File. . . . . . . . . . . . . . . . . . . . . . . . . . . . .25

2.1.11 The −d Option: Pre−Define a Macro. . . . . . . . . . . . . . . . . . . . . . . . . . .25

2.1.12 The −u Option: Undefine a Macro. . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

2.1.13 The −e Option: Preprocess Only. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .26

2.1.14 The −a Option: Don’t Preprocess At All. . . . . . . . . . . . . . . . . . . . . . . . .26

2.1.15 The −On Option: Specifying Multipass Optimization.. . . . . . . . . . . . . . . . . .26

2.1.16 The −t option: Enable TASM Compatibility Mode. . . . . . . . . . . . . . . . . . .27

2.1.17 The −w Option: Enable or Disable Assembly Warnings. . . . . . . . . . . . . . . . .27

2.1.18 The −v Option: Display Version Info. . . . . . . . . . . . . . . . . . . . . . . . . . .27

2.1.19 The NASMENV Environment Variable. . . . . . . . . . . . . . . . . . . . . . . . . . .28

2.2 Quick Start for MASM Users. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

3

2.2.1 NASM Is Case−Sensitive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .28

2.2.2 NASM Requires Square Brackets For Memory References. . . . . . . . . . . . . . . .28

2.2.3 NASM Doesn’t Store Variable Types. . . . . . . . . . . . . . . . . . . . . . . . . . . .29

2.2.4 NASM Doesn’t ASSUME. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

2.2.5 NASM Doesn’t Support Memory Models. . . . . . . . . . . . . . . . . . . . . . . . .29

2.2.6 Floating−Point Differences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

2.2.7 Other Differences. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29

Chapter 3: The NASM Language. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

3.1 Layout of a NASM Source Line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31

3.2 Pseudo−Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .32

3.2.1 DB and friends: Declaring Initialised Data. . . . . . . . . . . . . . . . . . . . . . . . .32

3.2.2 RESB and friends: Declaring Uninitialised Data. . . . . . . . . . . . . . . . . . . . . .32

3.2.3 INCBIN : Including External Binary Files. . . . . . . . . . . . . . . . . . . . . . . . .32

3.2.4 EQU: Defining Constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

3.2.5 TIMES: Repeating Instructions or Data. . . . . . . . . . . . . . . . . . . . . . . . . . .33

3.3 Effective Addresses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33

3.4 Constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

3.4.1 Numeric Constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34

3.4.2 Character Constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

3.4.3 String Constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

3.4.4 Floating−Point Constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35

3.5 Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

3.5.1 | : Bitwise OR Operator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

3.5.2 ̂ : Bitwise XOR Operator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

3.5.3 &: Bitwise AND Operator. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

3.5.4 << and >>: Bit Shift Operators. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

3.5.5 + and −: Addition and Subtraction Operators. . . . . . . . . . . . . . . . . . . . . . . .36

3.5.6 * , / , // , % and %%: Multiplication and Division. . . . . . . . . . . . . . . . . . . . . .36

3.5.7 Unary Operators: +, −, ~ and SEG . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36

3.6 SEG and WRT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

3.7 STRICT: Inhibiting Optimization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37

3.8 Critical Expressions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38

3.9 Local Labels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39

Chapter 4: The NASM Preprocessor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41

4.1 Single−Line Macros. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41

4

4.1.1 The Normal Way: %define . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41

4.1.2 Enhancing %define: %xdefine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42

4.1.3 Concatenating Single Line Macro Tokens: %+ . . . . . . . . . . . . . . . . . . . . . . .43

4.1.4 Undefining macros: %undef . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

4.1.5 Preprocessor Variables: %assign . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

4.2 String Handling in Macros: %strlen and %substr . . . . . . . . . . . . . . . . . . . . .44

4.2.1 String Length: %strlen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

4.2.2 Sub−strings: %substr . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

4.3 Multi−Line Macros: %macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

4.3.1 Overloading Multi−Line Macros. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .45

4.3.2 Macro−Local Labels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

4.3.3 Greedy Macro Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .46

4.3.4 Default Macro Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .47

4.3.5 %0: Macro Parameter Counter. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .48

4.3.6 %rotate : Rotating Macro Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . .48

4.3.7 Concatenating Macro Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .49

4.3.8 Condition Codes as Macro Parameters. . . . . . . . . . . . . . . . . . . . . . . . . . .50

4.3.9 Disabling Listing Expansion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

4.4 Conditional Assembly. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .50

4.4.1 %ifdef : Testing Single−Line Macro Existence. . . . . . . . . . . . . . . . . . . . . .51

4.4.2 ifmacro : Testing Multi−Line Macro Existence. . . . . . . . . . . . . . . . . . . . .51

4.4.3 %ifctx : Testing the Context Stack. . . . . . . . . . . . . . . . . . . . . . . . . . . .51

4.4.4 %if : Testing Arbitrary Numeric Expressions. . . . . . . . . . . . . . . . . . . . . . .52

4.4.5 %ifidn and %ifidni : Testing Exact Text Identity. . . . . . . . . . . . . . . . . . .52

4.4.6 %ifid , %ifnum , %ifstr : Testing Token Types. . . . . . . . . . . . . . . . . . . . .52

4.4.7 %error : Reporting User−Defined Errors. . . . . . . . . . . . . . . . . . . . . . . . .53

4.5 Preprocessor Loops: %rep . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

4.6 Including Other Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .54

4.7 The Context Stack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55

4.7.1 %push and %pop: Creating and Removing Contexts. . . . . . . . . . . . . . . . . . .55

4.7.2 Context−Local Labels. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55

4.7.3 Context−Local Single−Line Macros. . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

4.7.4 %repl : Renaming a Context. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

4.7.5 Example Use of the Context Stack: Block IFs. . . . . . . . . . . . . . . . . . . . . . .56

4.8 Standard Macros. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .58

5

4.8.1 __NASM_MAJOR__, __NASM_MINOR__, __NASM_SUBMINOR__ and___NASM_PATCHLEVEL__: NASM Version. . . . . . . . . . . . . . . . . . . . . . .58

4.8.2 __NASM_VERSION_ID__: NASM Version ID. . . . . . . . . . . . . . . . . . . . . .58

4.8.3 __NASM_VER__: NASM Version string. . . . . . . . . . . . . . . . . . . . . . . . . .58

4.8.4 __FILE__ and __LINE__ : File Name and Line Number. . . . . . . . . . . . . . . .58

4.8.5 STRUC and ENDSTRUC: Declaring Structure Data Types. . . . . . . . . . . . . . . . .59

4.8.6 ISTRUC, AT and IEND: Declaring Instances of Structures. . . . . . . . . . . . . . . .60

4.8.7 ALIGN and ALIGNB: Data Alignment. . . . . . . . . . . . . . . . . . . . . . . . . . .60

4.9 TASM Compatible Preprocessor Directives. . . . . . . . . . . . . . . . . . . . . . . . . . .61

4.9.1 %arg Directive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .61

4.9.2 %stacksize Directive. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62

4.9.3 %local Directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .62

4.10 Other Preprocessor Directives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63

4.10.1 %line Directive . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .63

4.10.2 %!<env> : Read an environment variable.. . . . . . . . . . . . . . . . . . . . . . . .63

Chapter 5: Assembler Directives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64

5.1 BITS : Specifying Target Processor Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . .64

5.1.1 USE16 & USE32: Aliases for BITS . . . . . . . . . . . . . . . . . . . . . . . . . . . .64

5.2 SECTION or SEGMENT: Changing and Defining Sections. . . . . . . . . . . . . . . . . . .64

5.2.1 The __SECT__ Macro . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .65

5.3 ABSOLUTE: Defining Absolute Labels. . . . . . . . . . . . . . . . . . . . . . . . . . . . .65

5.4 EXTERN: Importing Symbols from Other Modules. . . . . . . . . . . . . . . . . . . . . . .66

5.5 GLOBAL: Exporting Symbols to Other Modules. . . . . . . . . . . . . . . . . . . . . . . . .67

5.6 COMMON: Defining Common Data Areas. . . . . . . . . . . . . . . . . . . . . . . . . . . .67

5.7 CPU: Defining CPU Dependencies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .67

Chapter 6: Output Formats. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69

6.1 bin : Flat−Form Binary Output. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69

6.1.1 ORG: Binary File Program Origin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .69

6.1.2 bin Extensions to the SECTION Directive. . . . . . . . . . . . . . . . . . . . . . . . .69

6.1.3 Multisection support for the BIN format.. . . . . . . . . . . . . . . . . . . . . . .70

6.2 obj : Microsoft OMF Object Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .70

6.2.1 obj Extensions to the SEGMENT Directive. . . . . . . . . . . . . . . . . . . . . . . . .71

6.2.2 GROUP: Defining Groups of Segments. . . . . . . . . . . . . . . . . . . . . . . . . . .72

6.2.3 UPPERCASE: Disabling Case Sensitivity in Output. . . . . . . . . . . . . . . . . . . .72

6.2.4 IMPORT: Importing DLL Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . .72

6

6.2.5 EXPORT: Exporting DLL Symbols. . . . . . . . . . . . . . . . . . . . . . . . . . . . .73

6.2.6 ..start : Defining the Program Entry Point. . . . . . . . . . . . . . . . . . . . . . .73

6.2.7 obj Extensions to the EXTERN Directive . . . . . . . . . . . . . . . . . . . . . . . . .73

6.2.8 obj Extensions to the COMMON Directive . . . . . . . . . . . . . . . . . . . . . . . . .74

6.3 win32 : Microsoft Win32 Object Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .74

6.3.1 win32 Extensions to the SECTION Directive. . . . . . . . . . . . . . . . . . . . . . .75

6.4 coff : Common Object File Format. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .75

6.5 elf : Executable and Linkable Format Object Files. . . . . . . . . . . . . . . . . . . . . . .75

6.5.1 elf Extensions to the SECTION Directive. . . . . . . . . . . . . . . . . . . . . . . . .76

6.5.2 Position−Independent Code: elf Special Symbols and WRT. . . . . . . . . . . . . . .76

6.5.3 elf Extensions to the GLOBAL Directive . . . . . . . . . . . . . . . . . . . . . . . . .77

6.5.4 elf Extensions to the COMMON Directive . . . . . . . . . . . . . . . . . . . . . . . . .77

6.5.5 16−bit code and ELF . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78

6.6 aout : Linux a.out Object Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .78

6.7 aoutb : NetBSD/FreeBSD/OpenBSD a.out Object Files . . . . . . . . . . . . . . . . . .78

6.8 as86 : Minix/Linux as86 Object Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . .78

6.9 rdf : Relocatable Dynamic Object File Format. . . . . . . . . . . . . . . . . . . . . . . . .78

6.9.1 Requiring a Library: The LIBRARY Directive . . . . . . . . . . . . . . . . . . . . . . .79

6.9.2 Specifying a Module Name: The MODULE Directive. . . . . . . . . . . . . . . . . . . .79

6.9.3 rdf Extensions to the GLOBAL directive. . . . . . . . . . . . . . . . . . . . . . . . . .79

6.10 dbg : Debugging Format. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .79

Chapter 7: Writing 16−bit Code (DOS, Windows 3/3.1). . . . . . . . . . . . . . . . . . . . . . . .81

7.1 Producing .EXE Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .81

7.1.1 Using the obj Format To Generate .EXE Files . . . . . . . . . . . . . . . . . . . . . .81

7.1.2 Using the bin Format To Generate .EXE Files . . . . . . . . . . . . . . . . . . . . . .82

7.2 Producing .COM Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .83

7.2.1 Using the bin Format To Generate .COM Files . . . . . . . . . . . . . . . . . . . . . .83

7.2.2 Using the obj Format To Generate .COM Files . . . . . . . . . . . . . . . . . . . . . .84

7.3 Producing .SYS Files. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84

7.4 Interfacing to 16−bit C Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84

7.4.1 External Symbol Names. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .84

7.4.2 Memory Models. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .85

7.4.3 Function Definitions and Function Calls. . . . . . . . . . . . . . . . . . . . . . . . . .86

7.4.4 Accessing Data Items. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .88

7.4.5 c16.mac : Helper Macros for the 16−bit C Interface. . . . . . . . . . . . . . . . . . .89

7

7.5 Interfacing to Borland Pascal Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . .90

7.5.1 The Pascal Calling Convention. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .90

7.5.2 Borland Pascal Segment Name Restrictions. . . . . . . . . . . . . . . . . . . . . . . .91

7.5.3 Using c16.mac With Pascal Programs. . . . . . . . . . . . . . . . . . . . . . . . . .92

Chapter 8: Writing 32−bit Code (Unix, Win32, DJGPP). . . . . . . . . . . . . . . . . . . . . . . .93

8.1 Interfacing to 32−bit C Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93

8.1.1 External Symbol Names. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .93

8.1.2 Function Definitions and Function Calls. . . . . . . . . . . . . . . . . . . . . . . . . .93

8.1.3 Accessing Data Items. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .95

8.1.4 c32.mac : Helper Macros for the 32−bit C Interface. . . . . . . . . . . . . . . . . . .95

8.2 Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared Libraries. . . . . . . . . . . .96

8.2.1 Obtaining the Address of the GOT. . . . . . . . . . . . . . . . . . . . . . . . . . . . .96

8.2.2 Finding Your Local Data Items. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .97

8.2.3 Finding External and Common Data Items. . . . . . . . . . . . . . . . . . . . . . . . .98

8.2.4 Exporting Symbols to the Library User. . . . . . . . . . . . . . . . . . . . . . . . . . .98

8.2.5 Calling Procedures Outside the Library. . . . . . . . . . . . . . . . . . . . . . . . . . .99

8.2.6 Generating the Library File. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .99

Chapter 9: Mixing 16 and 32 Bit Code. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

9.1 Mixed−Size Jumps. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

9.2 Addressing Between Different−Size Segments. . . . . . . . . . . . . . . . . . . . . . . . 100

9.3 Other Mixed−Size Instructions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Chapter 10: Troubleshooting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

10.1 Common Problems. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

10.1.1 NASM Generates Inefficient Code. . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

10.1.2 My Jumps are Out of Range. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

10.1.3 ORG Doesn’t Work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103

10.1.4 TIMES Doesn’t Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

10.2 Bugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

Appendix A: Ndisasm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

A.1 Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

A.2 Getting Started: Installation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

A.3 Running NDISASM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

A.3.1 COM Files: Specifying an Origin. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

A.3.2 Code Following Data: Synchronisation. . . . . . . . . . . . . . . . . . . . . . . . . . 106

A.3.3 Mixed Code and Data: Automatic (Intelligent) Synchronisation . . . . . . . . . . . . 107

8

A.3.4 Other Options. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

A.4 Bugs and Improvements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

Appendix B: x86 Instruction Reference. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

B.1 Key to Operand Specifications. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

B.2 Key to Opcode Descriptions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

B.2.1 Register Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

B.2.2 Condition Codes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

B.2.3 SSE Condition Predicates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

B.2.4 Status Flags. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

B.2.5 Effective Address Encoding: ModR/M and SIB. . . . . . . . . . . . . . . . . . . . . 113

B.3 Key to Instruction Flags. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

B.4 x86 Instruction Set. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

B.4.1 AAA, AAS, AAM, AAD: ASCII Adjustments. . . . . . . . . . . . . . . . . . . . . . . . 115

B.4.2 ADC: Add with Carry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

B.4.3 ADD: Add Integers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

B.4.4 ADDPD: ADD Packed Double−Precision FP Values. . . . . . . . . . . . . . . . . . . 117

B.4.5 ADDPS: ADD Packed Single−Precision FP Values. . . . . . . . . . . . . . . . . . . 117

B.4.6 ADDSD: ADD Scalar Double−Precision FP Values. . . . . . . . . . . . . . . . . . . 117

B.4.7 ADDSS: ADD Scalar Single−Precision FP Values. . . . . . . . . . . . . . . . . . . . 117

B.4.8 AND: Bitwise AND . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

B.4.9 ANDNPD: Bitwise Logical AND NOT of Packed Double−Precision FP Values. . . . 118

B.4.10 ANDNPS: Bitwise Logical AND NOT of Packed Single−Precision FP Values. . . . 118

B.4.11 ANDPD: Bitwise Logical AND For Single FP. . . . . . . . . . . . . . . . . . . . . . 118

B.4.12 ANDPS: Bitwise Logical AND For Single FP. . . . . . . . . . . . . . . . . . . . . . 119

B.4.13 ARPL: Adjust RPL Field of Selector. . . . . . . . . . . . . . . . . . . . . . . . . . 119

B.4.14 BOUND: Check Array Index against Bounds. . . . . . . . . . . . . . . . . . . . . . 119

B.4.15 BSF, BSR: Bit Scan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

B.4.16 BSWAP: Byte Swap. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

B.4.17 BT, BTC, BTR, BTS: Bit Test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

B.4.18 CALL: Call Subroutine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120

B.4.19 CBW, CWD, CDQ, CWDE: Sign Extensions. . . . . . . . . . . . . . . . . . . . . . . . 121

B.4.20 CLC, CLD, CLI , CLTS: Clear Flags. . . . . . . . . . . . . . . . . . . . . . . . . . . 121

B.4.21 CLFLUSH: Flush Cache Line. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

B.4.22 CMC: Complement Carry Flag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

B.4.23 CMOVcc: Conditional Move. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

9

B.4.24 CMP: Compare Integers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122

B.4.25 CMPccPD: Packed Double−Precision FP Compare . . . . . . . . . . . . . . . . 122

B.4.26 CMPccPS: Packed Single−Precision FP Compare . . . . . . . . . . . . . . . . 123

B.4.27 CMPSB, CMPSW, CMPSD: Compare Strings. . . . . . . . . . . . . . . . . . . . . . . 124

B.4.28 CMPccSD: Scalar Double−Precision FP Compare . . . . . . . . . . . . . . . . 124

B.4.29 CMPccSS: Scalar Single−Precision FP Compare . . . . . . . . . . . . . . . . . 125

B.4.30 CMPXCHG, CMPXCHG486: Compare and Exchange. . . . . . . . . . . . . . . . . . 125

B.4.31 CMPXCHG8B: Compare and Exchange Eight Bytes. . . . . . . . . . . . . . . . . . 126

B.4.32 COMISD: Scalar Ordered Double−Precision FP Compare and Set EFLAGS. . . . . 126

B.4.33 COMISS: Scalar Ordered Single−Precision FP Compare and Set EFLAGS. . . . . . 126

B.4.34 CPUID: Get CPU Identification Code. . . . . . . . . . . . . . . . . . . . . . . . . . 127

B.4.35 CVTDQ2PD: Packed Signed INT32 to Packed Double−Precision FP Conversion. .127

B.4.36 CVTDQ2PS: Packed Signed INT32 to Packed Single−Precision FP Conversion. . . 127

B.4.37 CVTPD2DQ: Packed Double−Precision FP to Packed Signed INT32 Conversion. .127

B.4.38 CVTPD2PI: Packed Double−Precision FP to Packed Signed INT32 Conversion. .128

B.4.39 CVTPD2PS: Packed Double−Precision FP to Packed Single−Precision FPConversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128

B.4.40 CVTPI2PD: Packed Signed INT32 to Packed Double−Precision FP Conversion. .128

B.4.41 CVTPI2PS: Packed Signed INT32 to Packed Single−FP Conversion. . . . . . . . 128

B.4.42 CVTPS2DQ: Packed Single−Precision FP to Packed Signed INT32 Conversion. . . 128

B.4.43 CVTPS2PD: Packed Single−Precision FP to Packed Double−Precision FPConversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

B.4.44 CVTPS2PI: Packed Single−Precision FP to Packed Signed INT32 Conversion. . . 129

B.4.45 CVTSD2SI: Scalar Double−Precision FP to Signed INT32 Conversion. . . . . . . 129

B.4.46 CVTSD2SS: Scalar Double−Precision FP to Scalar Single−Precision FPConversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129

B.4.47 CVTSI2SD: Signed INT32 to Scalar Double−Precision FP Conversion. . . . . . . 129

B.4.48 CVTSI2SS: Signed INT32 to Scalar Single−Precision FP Conversion. . . . . . . . 130

B.4.49 CVTSS2SD: Scalar Single−Precision FP to Scalar Double−Precision FPConversion. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

B.4.50 CVTSS2SI: Scalar Single−Precision FP to Signed INT32 Conversion. . . . . . . . 130

B.4.51 CVTTPD2DQ: Packed Double−Precision FP to Packed Signed INT32 Conversionwith Truncation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

B.4.52 CVTTPD2PI: Packed Double−Precision FP to Packed Signed INT32 Conversionwith Truncation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130

B.4.53 CVTTPS2DQ: Packed Single−Precision FP to Packed Signed INT32 Conversionwith Truncation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

10

B.4.54 CVTTPS2PI: Packed Single−Precision FP to Packed Signed INT32 Conversionwith Truncation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

B.4.55 CVTTSD2SI: Scalar Double−Precision FP to Signed INT32 Conversion withTruncation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

B.4.56 CVTTSS2SI: Scalar Single−Precision FP to Signed INT32 Conversion withTruncation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

B.4.57 DAA, DAS: Decimal Adjustments. . . . . . . . . . . . . . . . . . . . . . . . . . . . 131

B.4.58 DEC: Decrement Integer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

B.4.59 DIV : Unsigned Integer Divide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132

B.4.60 DIVPD: Packed Double−Precision FP Divide. . . . . . . . . . . . . . . . . . . . . 132

B.4.61 DIVPS: Packed Single−Precision FP Divide. . . . . . . . . . . . . . . . . . . . . . 133

B.4.62 DIVSD: Scalar Double−Precision FP Divide. . . . . . . . . . . . . . . . . . . . . . 133

B.4.63 DIVSS: Scalar Single−Precision FP Divide. . . . . . . . . . . . . . . . . . . . . . 133

B.4.64 EMMS: Empty MMX State. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

B.4.65 ENTER: Create Stack Frame. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133

B.4.66 F2XM1: Calculate 2**X−1. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134

B.4.67 FABS: Floating−Point Absolute Value. . . . . . . . . . . . . . . . . . . . . . . . . 134

B.4.68 FADD, FADDP: Floating−Point Addition. . . . . . . . . . . . . . . . . . . . . . . . 134

B.4.69 FBLD, FBSTP: BCD Floating−Point Load and Store. . . . . . . . . . . . . . . . . 134

B.4.70 FCHS: Floating−Point Change Sign. . . . . . . . . . . . . . . . . . . . . . . . . . . 135

B.4.71 FCLEX, FNCLEX: Clear Floating−Point Exceptions. . . . . . . . . . . . . . . . . . 135

B.4.72 FCMOVcc: Floating−Point Conditional Move. . . . . . . . . . . . . . . . . . . . . 135

B.4.73 FCOM, FCOMP, FCOMPP, FCOMI, FCOMIP: Floating−Point Compare. . . . . . . . 136

B.4.74 FCOS: Cosine. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136

B.4.75 FDECSTP: Decrement Floating−Point Stack Pointer. . . . . . . . . . . . . . . . . . 136

B.4.76 FxDISI , FxENI : Disable and Enable Floating−Point Interrupts. . . . . . . . . . . 136

B.4.77 FDIV , FDIVP, FDIVR, FDIVRP: Floating−Point Division. . . . . . . . . . . . . . 137

B.4.78 FEMMS: Faster Enter/Exit of the MMX or floating−point state. . . . . . . . . . . . 137

B.4.79 FFREE: Flag Floating−Point Register as Unused. . . . . . . . . . . . . . . . . . . . 137

B.4.80 FIADD: Floating−Point/Integer Addition. . . . . . . . . . . . . . . . . . . . . . . . 138

B.4.81 FICOM, FICOMP: Floating−Point/Integer Compare. . . . . . . . . . . . . . . . . . 138

B.4.82 FIDIV , FIDIVR : Floating−Point/Integer Division. . . . . . . . . . . . . . . . . . 138

B.4.83 FILD , FIST , FISTP : Floating−Point/Integer Conversion. . . . . . . . . . . . . . 138

B.4.84 FIMUL: Floating−Point/Integer Multiplication. . . . . . . . . . . . . . . . . . . . . 138

B.4.85 FINCSTP: Increment Floating−Point Stack Pointer. . . . . . . . . . . . . . . . . . 138

B.4.86 FINIT , FNINIT : Initialise Floating−Point Unit. . . . . . . . . . . . . . . . . . . . 139

11

B.4.87 FISUB: Floating−Point/Integer Subtraction. . . . . . . . . . . . . . . . . . . . . . 139

B.4.88 FLD: Floating−Point Load. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139

B.4.89 FLDxx : Floating−Point Load Constants. . . . . . . . . . . . . . . . . . . . . . . . 139

B.4.90 FLDCW: Load Floating−Point Control Word. . . . . . . . . . . . . . . . . . . . . . 139

B.4.91 FLDENV: Load Floating−Point Environment. . . . . . . . . . . . . . . . . . . . . . 140

B.4.92 FMUL, FMULP: Floating−Point Multiply . . . . . . . . . . . . . . . . . . . . . . . . 140

B.4.93 FNOP: Floating−Point No Operation. . . . . . . . . . . . . . . . . . . . . . . . . . 140

B.4.94 FPATAN, FPTAN: Arctangent and Tangent. . . . . . . . . . . . . . . . . . . . . . . 140

B.4.95 FPREM, FPREM1: Floating−Point Partial Remainder. . . . . . . . . . . . . . . . . 140

B.4.96 FRNDINT: Floating−Point Round to Integer. . . . . . . . . . . . . . . . . . . . . . 141

B.4.97 FSAVE, FRSTOR: Save/Restore Floating−Point State. . . . . . . . . . . . . . . . . 141

B.4.98 FSCALE: Scale Floating−Point Value by Power of Two. . . . . . . . . . . . . . . . 141

B.4.99 FSETPM: Set Protected Mode. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

B.4.100 FSIN , FSINCOS: Sine and Cosine. . . . . . . . . . . . . . . . . . . . . . . . . . 141

B.4.101 FSQRT: Floating−Point Square Root. . . . . . . . . . . . . . . . . . . . . . . . . . 141

B.4.102 FST, FSTP: Floating−Point Store. . . . . . . . . . . . . . . . . . . . . . . . . . . 142

B.4.103 FSTCW: Store Floating−Point Control Word. . . . . . . . . . . . . . . . . . . . . 142

B.4.104 FSTENV: Store Floating−Point Environment. . . . . . . . . . . . . . . . . . . . . 142

B.4.105 FSTSW: Store Floating−Point Status Word. . . . . . . . . . . . . . . . . . . . . . 142

B.4.106 FSUB, FSUBP, FSUBR, FSUBRP: Floating−Point Subtract. . . . . . . . . . . . . 142

B.4.107 FTST: Test ST0 Against Zero. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

B.4.108 FUCOMxx: Floating−Point Unordered Compare. . . . . . . . . . . . . . . . . . . 143

B.4.109 FXAM: Examine Class of Value in ST0 . . . . . . . . . . . . . . . . . . . . . . . . 144

B.4.110 FXCH: Floating−Point Exchange. . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

B.4.111 FXRSTOR: Restore FP, MMX and SSE State. . . . . . . . . . . . . . . . . . . . . . 144

B.4.112 FXSAVE: Store FP, MMX and SSE State. . . . . . . . . . . . . . . . . . . . . . . . 144

B.4.113 FXTRACT: Extract Exponent and Significand. . . . . . . . . . . . . . . . . . . . . 144

B.4.114 FYL2X, FYL2XP1: Compute Y times Log2(X) or Log2(X+1). . . . . . . . . . . . 145

B.4.115 HLT: Halt Processor. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

B.4.116 IBTS : Insert Bit String. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

B.4.117 IDIV : Signed Integer Divide. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

B.4.118 IMUL: Signed Integer Multiply. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

B.4.119 IN : Input from I/O Port. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

B.4.120 INC: Increment Integer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 146

B.4.121 INSB, INSW, INSD: Input String from I/O Port. . . . . . . . . . . . . . . . . . . 147

12

B.4.122 INT : Software Interrupt. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147

B.4.123 INT3 , INT1 , ICEBP, INT01 : Breakpoints. . . . . . . . . . . . . . . . . . . . . . 147

B.4.124 INTO: Interrupt if Overflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

B.4.125 INVD: Invalidate Internal Caches. . . . . . . . . . . . . . . . . . . . . . . . . . . 148

B.4.126 INVLPG: Invalidate TLB Entry . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

B.4.127 IRET , IRETW, IRETD: Return from Interrupt. . . . . . . . . . . . . . . . . . . . 148

B.4.128 Jcc : Conditional Branch. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

B.4.129 JCXZ, JECXZ: Jump if CX/ECX Zero . . . . . . . . . . . . . . . . . . . . . . . . 148

B.4.130 JMP: Jump. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

B.4.131 LAHF: Load AH from Flags. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

B.4.132 LAR: Load Access Rights. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149

B.4.133 LDMXCSR: Load Streaming SIMD Extension Control/Status. . . . . . . . . . . . . 150

B.4.134 LDS, LES, LFS, LGS, LSS: Load Far Pointer. . . . . . . . . . . . . . . . . . . . . 150

B.4.135 LEA: Load Effective Address. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

B.4.136 LEAVE: Destroy Stack Frame. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150

B.4.137 LFENCE: Load Fence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151

B.4.138 LGDT, LIDT , LLDT: Load Descriptor Tables. . . . . . . . . . . . . . . . . . . . . 151

B.4.139 LMSW: Load/Store Machine Status Word. . . . . . . . . . . . . . . . . . . . . . . 151

B.4.140 LOADALL, LOADALL286: Load Processor State. . . . . . . . . . . . . . . . . . . 151

B.4.141 LODSB, LODSW, LODSD: Load from String. . . . . . . . . . . . . . . . . . . . . . 152

B.4.142 LOOP, LOOPE, LOOPZ, LOOPNE, LOOPNZ: Loop with Counter . . . . . . . . . . 152

B.4.143 LSL: Load Segment Limit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152

B.4.144 LTR: Load Task Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

B.4.145 MASKMOVDQU: Byte Mask Write . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

B.4.146 MASKMOVQ: Byte Mask Write. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153

B.4.147 MAXPD: Return Packed Double−Precision FP Maximum. . . . . . . . . . . . . . . 153

B.4.148 MAXPS: Return Packed Single−Precision FP Maximum. . . . . . . . . . . . . . . 153

B.4.149 MAXSD: Return Scalar Double−Precision FP Maximum. . . . . . . . . . . . . . . 153

B.4.150 MAXSS: Return Scalar Single−Precision FP Maximum. . . . . . . . . . . . . . . . 153

B.4.151 MFENCE: Memory Fence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154

B.4.152 MINPD: Return Packed Double−Precision FP Minimum. . . . . . . . . . . . . . . 154

B.4.153 MINPS: Return Packed Single−Precision FP Minimum. . . . . . . . . . . . . . . 154

B.4.154 MINSD: Return Scalar Double−Precision FP Minimum. . . . . . . . . . . . . . . 154

B.4.155 MINSS: Return Scalar Single−Precision FP Minimum. . . . . . . . . . . . . . . . 154

B.4.156 MOV: Move Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

13

B.4.157 MOVAPD: Move Aligned Packed Double−Precision FP Values. . . . . . . . . . . . 155

B.4.158 MOVAPS: Move Aligned Packed Single−Precision FP Values. . . . . . . . . . . . 156

B.4.159 MOVD: Move Doubleword to/from MMX Register. . . . . . . . . . . . . . . . . . 156

B.4.160 MOVDQ2Q: Move Quadword from XMM to MMX register.. . . . . . . . . . . . . 156

B.4.161 MOVDQA: Move Aligned Double Quadword. . . . . . . . . . . . . . . . . . . . . . 156

B.4.162 MOVDQU: Move Unaligned Double Quadword. . . . . . . . . . . . . . . . . . . . 156

B.4.163 MOVHLPS: Move Packed Single−Precision FP High to Low. . . . . . . . . . . . . 156

B.4.164 MOVHPD: Move High Packed Double−Precision FP. . . . . . . . . . . . . . . . . 157

B.4.165 MOVHPS: Move High Packed Single−Precision FP. . . . . . . . . . . . . . . . . . 157

B.4.166 MOVLHPS: Move Packed Single−Precision FP Low to High. . . . . . . . . . . . . 157

B.4.167 MOVLPD: Move Low Packed Double−Precision FP. . . . . . . . . . . . . . . . . . 157

B.4.168 MOVLPS: Move Low Packed Single−Precision FP. . . . . . . . . . . . . . . . . . 158

B.4.169 MOVMSKPD: Extract Packed Double−Precision FP Sign Mask. . . . . . . . . . . . 158

B.4.170 MOVMSKPS: Extract Packed Single−Precision FP Sign Mask. . . . . . . . . . . . 158

B.4.171 MOVNTDQ: Move Double Quadword Non Temporal. . . . . . . . . . . . . . . . . 158

B.4.172 MOVNTI: Move Doubleword Non Temporal. . . . . . . . . . . . . . . . . . . . . 158

B.4.173 MOVNTPD: Move Aligned Four Packed Single−Precision FP Values NonTemporal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 158

B.4.174 MOVNTPS: Move Aligned Four Packed Single−Precision FP Values NonTemporal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159

B.4.175 MOVNTQ: Move Quadword Non Temporal. . . . . . . . . . . . . . . . . . . . . . 159

B.4.176 MOVQ: Move Quadword to/from MMX Register. . . . . . . . . . . . . . . . . . . 159

B.4.177 MOVQ2DQ: Move Quadword from MMX to XMM register.. . . . . . . . . . . . . 159

B.4.178 MOVSB, MOVSW, MOVSD: Move String . . . . . . . . . . . . . . . . . . . . . . . . 159

B.4.179 MOVSD: Move Scalar Double−Precision FP Value. . . . . . . . . . . . . . . . . . 159

B.4.180 MOVSS: Move Scalar Single−Precision FP Value. . . . . . . . . . . . . . . . . . . 160

B.4.181 MOVSX, MOVZX: Move Data with Sign or Zero Extend. . . . . . . . . . . . . . . . 160

B.4.182 MOVUPD: Move Unaligned Packed Double−Precision FP Values. . . . . . . . . . 160

B.4.183 MOVUPS: Move Unaligned Packed Single−Precision FP Values. . . . . . . . . . . 160

B.4.184 MUL: Unsigned Integer Multiply. . . . . . . . . . . . . . . . . . . . . . . . . . . . 160

B.4.185 MULPD: Packed Single−FP Multiply. . . . . . . . . . . . . . . . . . . . . . . . . . 161

B.4.186 MULPS: Packed Single−FP Multiply. . . . . . . . . . . . . . . . . . . . . . . . . . 161

B.4.187 MULSD: Scalar Single−FP Multiply. . . . . . . . . . . . . . . . . . . . . . . . . . 161

B.4.188 MULSS: Scalar Single−FP Multiply. . . . . . . . . . . . . . . . . . . . . . . . . . 161

B.4.189 NEG, NOT: Two’s and One’s Complement. . . . . . . . . . . . . . . . . . . . . . . 161

14

B.4.190 NOP: No Operation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

B.4.191 OR: Bitwise OR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161

B.4.192 ORPD: Bit−wise Logical OR of Double−Precision FP Data. . . . . . . . . . . . . 162

B.4.193 ORPS: Bit−wise Logical OR of Single−Precision FP Data. . . . . . . . . . . . . . 162

B.4.194 OUT: Output Data to I/O Port. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 162

B.4.195 OUTSB, OUTSW, OUTSD: Output String to I/O Port. . . . . . . . . . . . . . . . . . 162

B.4.196 PACKSSDW, PACKSSWB, PACKUSWB: Pack Data. . . . . . . . . . . . . . . . . . 163

B.4.197 PADDB, PADDW, PADDD: Add Packed Integers. . . . . . . . . . . . . . . . . . . . 163

B.4.198 PADDQ: Add Packed Quadword Integers. . . . . . . . . . . . . . . . . . . . . . . 164

B.4.199 PADDSB, PADDSW: Add Packed Signed Integers With Saturation. . . . . . . . . . 164

B.4.200 PADDSIW: MMX Packed Addition to Implicit Destination. . . . . . . . . . . . . . 164

B.4.201 PADDUSB, PADDUSW: Add Packed Unsigned Integers With Saturation. . . . . . . 164

B.4.202 PAND, PANDN: MMX Bitwise AND and AND−NOT . . . . . . . . . . . . . . . . 164

B.4.203 PAUSE: Spin Loop Hint . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

B.4.204 PAVEB: MMX Packed Average. . . . . . . . . . . . . . . . . . . . . . . . . . . . 165

B.4.205 PAVGB PAVGW: Average Packed Integers. . . . . . . . . . . . . . . . . . . . . . . 165

B.4.206 PAVGUSB: Average of unsigned packed 8−bit values. . . . . . . . . . . . . . . . 165

B.4.207 PCMPxx: Compare Packed Integers.. . . . . . . . . . . . . . . . . . . . . . . . . . 165

B.4.208 PDISTIB : MMX Packed Distance and Accumulate with Implied Register. . . . . 166

B.4.209 PEXTRW: Extract Word. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166

B.4.210 PF2ID : Packed Single−Precision FP to Integer Convert. . . . . . . . . . . . . . . 167

B.4.211 PF2IW: Packed Single−Precision FP to Integer Word Convert. . . . . . . . . . . 167

B.4.212 PFACC: Packed Single−Precision FP Accumulate. . . . . . . . . . . . . . . . . . 167

B.4.213 PFADD: Packed Single−Precision FP Addition. . . . . . . . . . . . . . . . . . . . 167

B.4.214 PFCMPxx: Packed Single−Precision FP Compare . . . . . . . . . . . . . . . . . 167

B.4.215 PFMAX: Packed Single−Precision FP Maximum. . . . . . . . . . . . . . . . . . . 168

B.4.216 PFMIN: Packed Single−Precision FP Minimum. . . . . . . . . . . . . . . . . . . . 168

B.4.217 PFMUL: Packed Single−Precision FP Multiply. . . . . . . . . . . . . . . . . . . . 168

B.4.218 PFNACC: Packed Single−Precision FP Negative Accumulate. . . . . . . . . . . . 168

B.4.219 PFPNACC: Packed Single−Precision FP Mixed Accumulate. . . . . . . . . . . . . 168

B.4.220 PFRCP: Packed Single−Precision FP Reciprocal Approximation. . . . . . . . . . 168

B.4.221 PFRCPIT1: Packed Single−Precision FP Reciprocal, First Iteration Step. . . . . . 169

B.4.222 PFRCPIT2: Packed Single−Precision FP Reciprocal/ Reciprocal Square Root,Second Iteration Step. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

15

B.4.223 PFRSQIT1: Packed Single−Precision FP Reciprocal Square Root, FirstIteration Step. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169

B.4.224 PFRSQRT: Packed Single−Precision FP Reciprocal Square Root Approximation.169

B.4.225 PFSUB: Packed Single−Precision FP Subtract. . . . . . . . . . . . . . . . . . . . 169

B.4.226 PFSUBR: Packed Single−Precision FP Reverse Subtract. . . . . . . . . . . . . . . 169

B.4.227 PI2FD : Packed Doubleword Integer to Single−Precision FP Convert. . . . . . . . 170

B.4.228 PF2IW: Packed Word Integer to Single−Precision FP Convert. . . . . . . . . . . 170

B.4.229 PINSRW: Insert Word. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170

B.4.230 PMACHRIW: Packed Multiply and Accumulate with Rounding. . . . . . . . . . . 170

B.4.231 PMADDWD: MMX Packed Multiply and Add. . . . . . . . . . . . . . . . . . . . . 170

B.4.232 PMAGW: MMX Packed Magnitude. . . . . . . . . . . . . . . . . . . . . . . . . . . 171

B.4.233 PMAXSW: Packed Signed Integer Word Maximum. . . . . . . . . . . . . . . . . . 171

B.4.234 PMAXUB: Packed Unsigned Integer Byte Maximum. . . . . . . . . . . . . . . . . 171

B.4.235 PMINSW: Packed Signed Integer Word Minimum. . . . . . . . . . . . . . . . . . 171

B.4.236 PMINUB: Packed Unsigned Integer Byte Minimum. . . . . . . . . . . . . . . . . . 171

B.4.237 PMOVMSKB: Move Byte Mask To Integer. . . . . . . . . . . . . . . . . . . . . . . 171

B.4.238 PMULHRWC, PMULHRIW: Multiply Packed 16−bit Integers With Rounding, andStore High Word. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

B.4.239 PMULHRWA: Multiply Packed 16−bit Integers With Rounding, and Store HighWord. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

B.4.240 PMULHUW: Multiply Packed 16−bit Integers, and Store High Word. . . . . . . . . 172

B.4.241 PMULHW, PMULLW: Multiply Packed 16−bit Integers, and Store. . . . . . . . . . . 172

B.4.242 PMULUDQ: Multiply Packed Unsigned 32−bit Integers, and Store.. . . . . . . . . . 173

B.4.243 PMVccZB: MMX Packed Conditional Move. . . . . . . . . . . . . . . . . . . . . 173

B.4.244 POP: Pop Data from Stack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173

B.4.245 POPAx: Pop All General−Purpose Registers. . . . . . . . . . . . . . . . . . . . . 174

B.4.246 POPFx: Pop Flags Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

B.4.247 POR: MMX Bitwise OR . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174

B.4.248 PREFETCH: Prefetch Data Into Caches. . . . . . . . . . . . . . . . . . . . . . . . 174

B.4.249 PREFETCHh: Prefetch Data Into Caches . . . . . . . . . . . . . . . . . . . . . . 175

B.4.250 PSADBW: Packed Sum of Absolute Differences. . . . . . . . . . . . . . . . . . . . 175

B.4.251 PSHUFD: Shuffle Packed Doublewords. . . . . . . . . . . . . . . . . . . . . . . . 175

B.4.252 PSHUFHW: Shuffle Packed High Words. . . . . . . . . . . . . . . . . . . . . . . . 175

B.4.253 PSHUFLW: Shuffle Packed Low Words. . . . . . . . . . . . . . . . . . . . . . . . 176

B.4.254 PSHUFW: Shuffle Packed Words. . . . . . . . . . . . . . . . . . . . . . . . . . . . 176

B.4.255 PSLLx: Packed Data Bit Shift Left Logical. . . . . . . . . . . . . . . . . . . . . . 176

16

B.4.256 PSRAx: Packed Data Bit Shift Right Arithmetic. . . . . . . . . . . . . . . . . . . 176

B.4.257 PSRLx: Packed Data Bit Shift Right Logical. . . . . . . . . . . . . . . . . . . . . 177

B.4.258 PSUBx: Subtract Packed Integers. . . . . . . . . . . . . . . . . . . . . . . . . . . 177

B.4.259 PSUBSxx, PSUBUSx: Subtract Packed Integers With Saturation. . . . . . . . . . 178

B.4.260 PSUBSIW: MMX Packed Subtract with Saturation to Implied Destination. . . . . 178

B.4.261 PSWAPD: Swap Packed Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

B.4.262 PUNPCKxxx: Unpack and Interleave Data. . . . . . . . . . . . . . . . . . . . . . 179

B.4.263 PUSH: Push Data on Stack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

B.4.264 PUSHAx: Push All General−Purpose Registers. . . . . . . . . . . . . . . . . . . . 180

B.4.265 PUSHFx: Push Flags Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 180

B.4.266 PXOR: MMX Bitwise XOR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181

B.4.267 RCL, RCR: Bitwise Rotate through Carry Bit. . . . . . . . . . . . . . . . . . . . . 181

B.4.268 RCPPS: Packed Single−Precision FP Reciprocal. . . . . . . . . . . . . . . . . . . 181

B.4.269 RCPSS: Scalar Single−Precision FP Reciprocal. . . . . . . . . . . . . . . . . . . . 182

B.4.270 RDMSR: Read Model−Specific Registers. . . . . . . . . . . . . . . . . . . . . . . 182

B.4.271 RDPMC: Read Performance−Monitoring Counters. . . . . . . . . . . . . . . . . . 182

B.4.272 RDSHR: Read SMM Header Pointer Register. . . . . . . . . . . . . . . . . . . . . 182

B.4.273 RDTSC: Read Time−Stamp Counter. . . . . . . . . . . . . . . . . . . . . . . . . . 182

B.4.274 RET, RETF, RETN: Return from Procedure Call. . . . . . . . . . . . . . . . . . . 182

B.4.275 ROL, ROR: Bitwise Rotate. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182

B.4.276 RSDC: Restore Segment Register and Descriptor. . . . . . . . . . . . . . . . . . . 183

B.4.277 RSLDT: Restore Segment Register and Descriptor. . . . . . . . . . . . . . . . . . 183

B.4.278 RSM: Resume from System−Management Mode. . . . . . . . . . . . . . . . . . . 183

B.4.279 RSQRTPS: Packed Single−Precision FP Square Root Reciprocal. . . . . . . . . . 183

B.4.280 RSQRTSS: Scalar Single−Precision FP Square Root Reciprocal. . . . . . . . . . . 183

B.4.281 RSTS: Restore TSR and Descriptor. . . . . . . . . . . . . . . . . . . . . . . . . . 184

B.4.282 SAHF: Store AH to Flags. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

B.4.283 SAL, SAR: Bitwise Arithmetic Shifts . . . . . . . . . . . . . . . . . . . . . . . . . 184

B.4.284 SALC: Set AL from Carry Flag. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184

B.4.285 SBB: Subtract with Borrow. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185

B.4.286 SCASB, SCASW, SCASD: Scan String. . . . . . . . . . . . . . . . . . . . . . . . . 185

B.4.287 SETcc: Set Register from Condition. . . . . . . . . . . . . . . . . . . . . . . . . 185

B.4.288 SFENCE: Store Fence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186

B.4.289 SGDT, SIDT , SLDT: Store Descriptor Table Pointers. . . . . . . . . . . . . . . . 186

B.4.290 SHL, SHR: Bitwise Logical Shifts. . . . . . . . . . . . . . . . . . . . . . . . . . . 186

17

B.4.291 SHLD, SHRD: Bitwise Double−Precision Shifts. . . . . . . . . . . . . . . . . . . . 187

B.4.292 SHUFPD: Shuffle Packed Double−Precision FP Values. . . . . . . . . . . . . . . 187

B.4.293 SHUFPS: Shuffle Packed Single−Precision FP Values. . . . . . . . . . . . . . . . 188

B.4.294 SMI: System Management Interrupt. . . . . . . . . . . . . . . . . . . . . . . . . . 188

B.4.295 SMINT, SMINTOLD: Software SMM Entry (CYRIX). . . . . . . . . . . . . . . . 188

B.4.296 SMSW: Store Machine Status Word. . . . . . . . . . . . . . . . . . . . . . . . . . . 188

B.4.297 SQRTPD: Packed Double−Precision FP Square Root. . . . . . . . . . . . . . . . . 188

B.4.298 SQRTPS: Packed Single−Precision FP Square Root. . . . . . . . . . . . . . . . . 188

B.4.299 SQRTSD: Scalar Double−Precision FP Square Root. . . . . . . . . . . . . . . . . 188

B.4.300 SQRTSS: Scalar Single−Precision FP Square Root. . . . . . . . . . . . . . . . . . 189

B.4.301 STC, STD, STI : Set Flags. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

B.4.302 STMXCSR: Store Streaming SIMD Extension Control/Status. . . . . . . . . . . . 189

B.4.303 STOSB, STOSW, STOSD: Store Byte to String. . . . . . . . . . . . . . . . . . . . 189

B.4.304 STR: Store Task Register. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189

B.4.305 SUB: Subtract Integers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 190

B.4.306 SUBPD: Packed Double−Precision FP Subtract. . . . . . . . . . . . . . . . . . . . 190

B.4.307 SUBPS: Packed Single−Precision FP Subtract. . . . . . . . . . . . . . . . . . . . 190

B.4.308 SUBSD: Scalar Single−FP Subtract. . . . . . . . . . . . . . . . . . . . . . . . . . 190

B.4.309 SUBSS: Scalar Single−FP Subtract. . . . . . . . . . . . . . . . . . . . . . . . . . 190

B.4.310 SVDC: Save Segment Register and Descriptor. . . . . . . . . . . . . . . . . . . . . 191

B.4.311 SVLDT: Save LDTR and Descriptor. . . . . . . . . . . . . . . . . . . . . . . . . . 191

B.4.312 SVTS: Save TSR and Descriptor. . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

B.4.313 SYSCALL: Call Operating System. . . . . . . . . . . . . . . . . . . . . . . . . . . 191

B.4.314 SYSENTER: Fast System Call. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

B.4.315 SYSEXIT: Fast Return From System Call. . . . . . . . . . . . . . . . . . . . . . . 192

B.4.316 SYSRET: Return From Operating System. . . . . . . . . . . . . . . . . . . . . . . 192

B.4.317 TEST: Test Bits (notional bitwise AND). . . . . . . . . . . . . . . . . . . . . . . 193

B.4.318 UCOMISD: Unordered Scalar Double−Precision FP compare and set EFLAGS. . .193

B.4.319 UCOMISS: Unordered Scalar Single−Precision FP compare and set EFLAGS. . .193

B.4.320 UD0, UD1, UD2: Undefined Instruction. . . . . . . . . . . . . . . . . . . . . . . . 193

B.4.321 UMOV: User Move Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194

B.4.322 UNPCKHPD: Unpack and Interleave High Packed Double−Precision FP Values. .194

B.4.323 UNPCKHPS: Unpack and Interleave High Packed Single−Precision FP Values. . . 194

B.4.324 UNPCKLPD: Unpack and Interleave Low Packed Double−Precision FP Data. . . . 194

B.4.325 UNPCKLPS: Unpack and Interleave Low Packed Single−Precision FP Data. . . . 194

18

B.4.326 VERR, VERW: Verify Segment Readability/Writability. . . . . . . . . . . . . . . . 195

B.4.327 WAIT: Wait for Floating−Point Processor. . . . . . . . . . . . . . . . . . . . . . . 195

B.4.328 WBINVD: Write Back and Invalidate Cache. . . . . . . . . . . . . . . . . . . . . . 195

B.4.329 WRMSR: Write Model−Specific Registers. . . . . . . . . . . . . . . . . . . . . . . 195

B.4.330 WRSHR: Write SMM Header Pointer Register. . . . . . . . . . . . . . . . . . . . . 195

B.4.331 XADD: Exchange and Add. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195

B.4.332 XBTS: Extract Bit String. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

B.4.333 XCHG: Exchange. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

B.4.334 XLATB: Translate Byte in Lookup Table. . . . . . . . . . . . . . . . . . . . . . . 196

B.4.335 XOR: Bitwise Exclusive OR. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196

B.4.336 XORPD: Bitwise Logical XOR of Double−Precision FP Values. . . . . . . . . . . 197

B.4.337 XORPS: Bitwise Logical XOR of Single−Precision FP Values. . . . . . . . . . . . 197

19

Chapter 1: Introduction

1.1 What Is NASM?The Netwide Assembler, NASM, is an 80x86 assembler designed for portability and modularity. Itsupports a range of object file formats, including Linux and NetBSD/FreeBSD a.out , ELF,COFF, Microsoft 16−bit OBJ and Win32 . It will also output plain binary files. Its syntax isdesigned to be simple and easy to understand, similar to Intel’s but less complex. It supportsPentium , P6, MMX, 3DNow!, SSE and SSE2 opcodes, and has macro capability.

1.1.1 Why Yet Another Assembler?The Netwide Assembler grew out of an idea on comp.lang.asm.x86 (or possiblyalt.lang.asm – I forget which), which was essentially that there didn’t seem to be a good freex86−series assembler around, and that maybe someone ought to write one.

• a86 is good, but not free, and in particular you don’t get any 32−bit capability until you pay. It’sDOS only, too.

• gas is free, and ports over DOS and Unix, but it’s not very good, since it’s designed to be a backend to gcc , which always feeds it correct code. So its error checking is minimal. Also, its syntaxis horrible, from the point of view of anyone trying to actually write anything in it. Plus you can’twrite 16−bit code in it (properly).

• as86 is Minix− and Linux−specific, and (my version at least) doesn’t seem to have much (orany) documentation.

• MASM isn’t very good, and it’s expensive, and it runs only under DOS.

• TASM is better, but still strives for MASM compatibility, which means millions of directives andtons of red tape. And its syntax is essentially MASM’s, with the contradictions and quirks thatentails (although it sorts out some of those by means of Ideal mode). It’s expensive too. And it’sDOS−only.

So here, for your coding pleasure, is NASM. At present it’s still in prototype stage – we don’tpromise that it can outperform any of these assemblers. But please, please send us bug reports,fixes, helpful information, and anything else you can get your hands on (and thanks to the manypeople who’ve done this already! You all know who you are), and we’ll improve it out of allrecognition. Again.

1.1.2 Licence ConditionsPlease see the file Licence , supplied as part of any NASM distribution archive, for the licenceconditions under which you may use NASM.

1.2 Contact InformationThe current version of NASM (since about 0.98.08) are maintained by a team of developers,accessible through the nasm−devel mailing list (see below for the link). If you want to report abug, please read section 10.2 first.

NASM has a WWW page at http://www.web−sites.co.uk/nasm , and another, withadditional information, at http://nasm.2y.net/

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http://nasm.2y.net/

http://www.web-sites.co.uk/nasm

The original authors are e−mailable as [email protected] and [email protected] . Thelatter is no longer involved in the development team.

New releases of NASM are uploaded to the official siteshttp://www.web−sites.co.uk/nasm and to ftp.kernel.org and ibiblio.org .

Announcements are posted to comp.lang.asm.x86 , alt.lang.asm andcomp.os.linux.announce

If you want information about NASM beta releases, and the current development status, pleasesubscribe to the nasm−devel email list by registering athttp://groups.yahoo.com/group/nasm−devel ,http://www.pairlist.net/mailman/listinfo/nasm−devel andhttp://sourceforge.net/projects/nasm .

The preferred list is the list at Sourceforge, which is also the home to the latest nasm source codeand releases. The other lists are open, but may not continue to be supported in the long term.

1.3 Installation1.3.1 Installing NASM under MS−DOS or Windows

Once you’ve obtained the DOS archive for NASM, nasmXXX.zip (where XXX denotes theversion number of NASM contained in the archive), unpack it into its own directory (for examplec:\nasm ).

The archive will contain four executable files: the NASM executable files nasm.exe andnasmw.exe , and the NDISASM executable files ndisasm.exe and ndisasmw.exe . In eachcase, the file whose name ends in w is a Win32 executable, designed to run under Windows 95 or Windows NT Intel, and the other one is a 16−bit DOS executable.

The only file NASM needs to run is its own executable, so copy (at least) one of nasm.exe andnasmw.exe to a directory on your PATH, or alternatively edit autoexec.bat to add the nasmdirectory to your PATH. (If you’re only installing the Win32 version, you may wish to rename it tonasm.exe .)

That’s it – NASM is installed. You don’t need the nasm directory to be present to run NASM(unless you’ve added it to your PATH), so you can delete it if you need to save space; however, youmay want to keep the documentation or test programs.

If you’ve downloaded the DOS source archive, nasmXXXs.zip , the nasm directory will alsocontain the full NASM source code, and a selection of Makefiles you can (hopefully) use to rebuildyour copy of NASM from scratch.

Note that the source files insnsa.c , insnsd.c , insnsi.h and insnsn.c are automaticallygenerated from the master instruction table insns.dat by a Perl script; the file macros.c isgenerated from standard.mac by another Perl script. Although the NASM 0.98 distributionincludes these generated files, you will need to rebuild them (and hence, will need a Perl interpreter)if you change insns.dat, standard.mac or the documentation. It is possible future source distributionsmay not include these files at all. Ports of Perl for a variety of platforms, including DOS andWindows, are available from www.cpan.org.

1.3.2 Installing NASM under UnixOnce you’ve obtained the Unix source archive for NASM, nasm−X.XX.tar.gz (where X.XXdenotes the version number of NASM contained in the archive), unpack it into a directory such as/usr/local/src . The archive, when unpacked, will create its own subdirectory nasm−X.XX .

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http://www.cpan.org/ports/

http://sourceforge.net/projects/nasm

http://www.pairlist.net/mailman/listinfo/nasm-devel

http://groups.yahoo.com/group/nasm-devel

news:comp.os.linux.announce

news:alt.lang.asm

news:comp.lang.asm.x86

ftp://ibiblio.org/pub/Linux/devel/lang/assemblers/

ftp://ftp.kernel.org/pub/software/devel/nasm/

http://www.web-sites.co.uk/nasm

mailto:[email protected]


NASM is an auto−configuring package: once you’ve unpacked it, cd to the directory it’s beenunpacked into and type ./configure . This shell script will find the best C compiler to use forbuilding NASM and set up Makefiles accordingly.

Once NASM has auto−configured, you can type make to build the nasm and ndisasm binaries,and then make install to install them in /usr/local/bin and install the man pagesnasm.1 and ndisasm.1 in /usr/local/man/man1 . Alternatively, you can give optionssuch as −−prefix to the configure script (see the file INSTALL for more details), or install theprograms yourself.

NASM also comes with a set of utilities for handling the RDOFF custom object−file format, whichare in the rdoff subdirectory of the NASM archive. You can build these with make rdf andinstall them with make rdf_install , if you want them.

If NASM fails to auto−configure, you may still be able to make it compile by using the fall−backUnix makefile Makefile.unx . Copy or rename that file to Makefile and try typing make.There is also a Makefile.unx file in the rdoff subdirectory.

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Chapter 2: Running NASM

2.1 NASM Command−Line SyntaxTo assemble a file, you issue a command of the form

nasm −f <format> <filename> [−o <output>]

For example,

nasm −f elf myfile.asm

will assemble myfile.asm into an ELF object file myfile.o . And

nasm −f bin myfile.asm −o myfile.com

will assemble myfile.asm into a raw binary file myfile.com .

To produce a listing file, with the hex codes output from NASM displayed on the left of the originalsources, use the −l option to give a listing file name, for example:

nasm −f coff myfile.asm −l myfile.lst

To get further usage instructions from NASM, try typing

nasm −h

This will also list the available output file formats, and what they are.

If you use Linux but aren’t sure whether your system is a.out or ELF, type

file nasm

(in the directory in which you put the NASM binary when you installed it). If it says something like

nasm: ELF 32−bit LSB executable i386 (386 and up) Version 1

then your system is ELF, and you should use the option −f elf when you want NASM to produceLinux object files. If it says

nasm: Linux/i386 demand−paged executable (QMAGIC)

or something similar, your system is a.out , and you should use −f aout instead (Linux a.outsystems have long been obsolete, and are rare these days.)

Like Unix compilers and assemblers, NASM is silent unless it goes wrong: you won’t see anyoutput at all, unless it gives error messages.

2.1.1 The −o Option: Specifying the Output File NameNASM will normally choose the name of your output file for you; precisely how it does this isdependent on the object file format. For Microsoft object file formats (obj and win32 ), it willremove the .asm extension (or whatever extension you like to use – NASM doesn’t care) fromyour source file name and substitute .obj . For Unix object file formats (aout , coff , elf andas86 ) it will substitute .o . For rdf , it will use .rdf , and for the bin format it will simplyremove the extension, so that myfile.asm produces the output file myfile .

If the output file already exists, NASM will overwrite it, unless it has the same name as the inputfile, in which case it will give a warning and use nasm.out as the output file name instead.

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For situations in which this behaviour is unacceptable, NASM provides the −o command−lineoption, which allows you to specify your desired output file name. You invoke −o by following itwith the name you wish for the output file, either with or without an intervening space. For example:

nasm −f bin program.asm −o program.com nasm −f bin driver.asm −odriver.sys

Note that this is a small o, and is different from a capital O , which is used to specify the number ofoptimisation passes required. See section 2.1.15.

2.1.2 The −f Option: Specifying the Output File FormatIf you do not supply the −f option to NASM, it will choose an output file format for you itself. Inthe distribution versions of NASM, the default is always bin ; if you’ve compiled your own copy ofNASM, you can redefine OF_DEFAULT at compile time and choose what you want the default tobe.

Like −o, the intervening space between −f and the output file format is optional; so −f elf and−felf are both valid.

A complete list of the available output file formats can be given by issuing the commandnasm −hf .

2.1.3 The −l Option: Generating a Listing FileIf you supply the −l option to NASM, followed (with the usual optional space) by a file name,NASM will generate a source−listing file for you, in which addresses and generated code are listedon the left, and the actual source code, with expansions of multi−line macros (except those whichspecifically request no expansion in source listings: see section 4.3.9) on the right. For example:

nasm −f elf myfile.asm −l myfile.lst

2.1.4 The −M Option: Generate Makefile Dependencies.This option can be used to generate makefile dependencies on stdout. This can be redirected to a filefor further processing. For example:

NASM −M myfile.asm > myfile.dep

2.1.5 The −F Option: Selecting a Debugging FormatThis option can be used to select a debugging format for the output file. The syntax is the same asfor the –f option, except that it produces output in a debugging format.

A complete list of the available debug file formats for an output format can be seen by issuing thecommand nasm −f <format> −y .

This option is not built into NASM by default. For information on how to enable it when buildingfrom the sources, see section 6.10

2.1.6 The −g Option: Enabling Debug Information.This option can be used to generate debugging information in the specified format.

See section 2.1.5 for more information.

2.1.7 The −E Option: Send Errors to a FileUnder MS−DOS it can be difficult (though there are ways) to redirect the standard−error output of aprogram to a file. Since NASM usually produces its warning and error messages on stderr , thiscan make it hard to capture the errors if (for example) you want to load them into an editor.

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NASM therefore provides the −E option, taking a filename argument which causes errors to be sentto the specified files rather than standard error. Therefore you can redirect the errors into a file bytyping

nasm −E myfile.err −f obj myfile.asm

2.1.8 The −s Option: Send Errors to stdout

The −s option redirects error messages to stdout rather than stderr , so it can be redirectedunder MS−DOS. To assemble the file myfile.asm and pipe its output to the more program, youcan type:

nasm −s −f obj myfile.asm | more

See also the −E option, section 2.1.7.

2.1.9 The −i Option: Include File Search DirectoriesWhen NASM sees the %include directive in a source file (see section 4.6), it will search for thegiven file not only in the current directory, but also in any directories specified on the command lineby the use of the −i option. Therefore you can include files from a macro library, for example, bytyping

nasm −ic:\\macrolib\\ −f obj myfile.asm

(As usual, a space between −i and the path name is allowed, and optional).

NASM, in the interests of complete source−code portability, does not understand the file namingconventions of the OS it is running on; the string you provide as an argument to the −i option willbe prepended exactly as written to the name of the include file. Therefore the trailing backslash inthe above example is necessary. Under Unix, a trailing forward slash is similarly necessary.

(You can use this to your advantage, if you’re really perverse, by noting that the option −ifoo willcause %include "bar.i" to search for the file foobar.i ...)

If you want to define a standard include search path, similar to /usr/include on Unix systems,you should place one or more −i directives in the NASMENV environment variable (see section2.1.19).

For Makefile compatibility with many C compilers, this option can also be specified as −I .

2.1.10 The −p Option: Pre−Include a FileNASM allows you to specify files to be pre−included into your source file, by the use of the −poption. So running

nasm myfile.asm −p myinc.inc

is equivalent to running nasm myfile.asm and placing the directive%include "myinc.inc" at the start of the file.

For consistency with the −I , −D and −U options, this option can also be specified as −P.

2.1.11 The −d Option: Pre−Define a MacroJust as the −p option gives an alternative to placing %include directives at the start of a sourcefile, the −d option gives an alternative to placing a %define directive. You could code

nasm myfile.asm −dFOO=100

as an alternative to placing the directive

%define FOO 100

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at the start of the file. You can miss off the macro value, as well: the option −dFOO is equivalent tocoding %define FOO . This form of the directive may be useful for selecting assembly−timeoptions which are then tested using %ifdef , for example −dDEBUG.

For Makefile compatibility with many C compilers, this option can also be specified as −D.

2.1.12 The −u Option: Undefine a MacroThe −u option undefines a macro that would otherwise have been pre−defined, either automaticallyor by a −p or −d option specified earlier on the command lines.

For example, the following command line:

nasm myfile.asm −dFOO=100 −uFOO

would result in FOO not being a predefined macro in the program. This is useful to override optionsspecified at a different point in a Makefile.

For Makefile compatibility with many C compilers, this option can also be specified as −U.

2.1.13 The −e Option: Preprocess OnlyNASM allows the preprocessor to be run on its own, up to a point. Using the −e option (whichrequires no arguments) will cause NASM to preprocess its input file, expand all the macroreferences, remove all the comments and preprocessor directives, and print the resulting file onstandard output (or save it to a file, if the −o option is also used).

This option cannot be applied to programs which require the preprocessor to evaluate expressionswhich depend on the values of symbols: so code such as

%assign tablesize ($−tablestart)

will cause an error in preprocess−only mode.

2.1.14 The −a Option: Don’t Preprocess At AllIf NASM is being used as the back end to a compiler, it might be desirable to suppresspreprocessing completely and assume the compiler has already done it, to save time and increasecompilation speeds. The −a option, requiring no argument, instructs NASM to replace its powerfulpreprocessor with a stub preprocessor which does nothing.

2.1.15 The −On Option: Specifying Multipass Optimization.NASM defaults to being a two pass assembler. This means that if you have a complex source filewhich needs more than 2 passes to assemble correctly, you have to tell it.

Using the −O option, you can tell NASM to carry out multiple passes. The syntax is:

• −O0 strict two−pass assembly, JMP and Jcc are handled more like v0.98, except that backwardJMPs are short, if possible. Immediate operands take their long forms if a short form is notspecified.

• −O1 strict two−pass assembly, but forward branches are assembled with code guaranteed toreach; may produce larger code than –O0, but will produce successful assembly more often ifbranch offset sizes are not specified. Additionally, immediate operands which will fit in a signedbyte are optimised, unless the long form is specified.

• −On multi−pass optimization, minimize branch offsets; also will minimize signed immediatebytes, overriding size specification when the strict keyword hasn’t been used (see section3.7). If 2 <= n <= 3, then there are 5 * n passes, otherwise there are n passes.

Note that this is a capital O, and is different from a small o, which is used to specify the outputformat. See section 2.1.1.

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2.1.16 The −t option: Enable TASM Compatibility ModeNASM includes a limited form of compatibility with Borland’s TASM. When NASM’s −t option isused, the following changes are made:

• local labels may be prefixed with @@ instead of .

• TASM−style response files beginning with @ may be specified on the command line. This isdifferent from the −@resp style that NASM natively supports.

• size override is supported within brackets. In TASM compatible mode, a size override insidesquare brackets changes the size of the operand, and not the address type of the operand as itdoes in NASM syntax. E.g. mov eax,[DWORD val] is valid syntax in TASM compatibilitymode. Note that you lose the ability to override the default address type for the instruction.

• %arg preprocessor directive is supported which is similar to TASM’s ARG directive.

• %local preprocessor directive

• %stacksize preprocessor directive

• unprefixed forms of some directives supported (arg , elif , else , endif , if , ifdef ,ifdifi , ifndef , include , local )

• more...

For more information on the directives, see the section on TASM Compatiblity preprocessordirectives in section 4.9.

2.1.17 The −w Option: Enable or Disable Assembly WarningsNASM can observe many conditions during the course of assembly which are worth mentioning tothe user, but not a sufficiently severe error to justify NASM refusing to generate an output file.These conditions are reported like errors, but come up with the word ‘warning’ before the message.Warnings do not prevent NASM from generating an output file and returning a success status to theoperating system.

Some conditions are even less severe than that: they are only sometimes worth mentioning to theuser. Therefore NASM supports the −w command−line option, which enables or disables certainclasses of assembly warning. Such warning classes are described by a name, for exampleorphan−labels ; you can enable warnings of this class by the command−line option−w+orphan−labels and disable it by −w−orphan−labels .

The suppressible warning classes are:

• macro−params covers warnings about multi−line macros being invoked with the wrongnumber of parameters. This warning class is enabled by default; see section 4.3.1 for an exampleof why you might want to disable it.

• orphan−labels covers warnings about source lines which contain no instruction but define alabel without a trailing colon. NASM does not warn about this somewhat obscure condition bydefault; see section 3.1 for an example of why you might want it to.

• number−overflow covers warnings about numeric constants which don’t fit in 32 bits (forexample, it’s easy to type one too many Fs and produce 0x7ffffffff by mistake). Thiswarning class is enabled by default.

2.1.18 The −v Option: Display Version InfoTyping NASM −v will display the version of NASM which you are using, and the date on which itwas compiled.

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You will need the version number if you report a bug.

2.1.19 The NASMENV Environment VariableIf you define an environment variable called NASMENV, the program will interpret it as a list ofextra command−line options, which are processed before the real command line. You can use this todefine standard search directories for include files, by putting −i options in the NASMENV variable.

The value of the variable is split up at white space, so that the value −s −ic:\nasmlib will betreated as two separate options. However, that means that the value −dNAME="my name" won’tdo what you might want, because it will be split at the space and the NASM command−lineprocessing will get confused by the two nonsensical words −dNAME="my and name" .

To get round this, NASM provides a feature whereby, if you begin the NASMENV environmentvariable with some character that isn’t a minus sign, then NASM will treat this character as theseparator character for options. So setting the NASMENV variable to the value!−s!−ic:\nasmlib is equivalent to setting it to −s −ic:\nasmlib , but!−dNAME="my name" will work.

This environment variable was previously called NASM. This was changed with version 0.98.31.

2.2 Quick Start for MASM UsersIf you’re used to writing programs with MASM, or with TASM in MASM−compatible (non−Ideal)mode, or with a86 , this section attempts to outline the major differences between MASM’s syntaxand NASM’s. If you’re not already used to MASM, it’s probably worth skipping this section.

2.2.1 NASM Is Case−SensitiveOne simple difference is that NASM is case−sensitive. It makes a difference whether you call yourlabel foo , Foo or FOO. If you’re assembling to DOS or OS/2 .OBJ files, you can invoke theUPPERCASE directive (documented in section 6.2) to ensure that all symbols exported to othercode modules are forced to be upper case; but even then, within a single module, NASM willdistinguish between labels differing only in case.

2.2.2 NASM Requires Square Brackets For Memory ReferencesNASM was designed with simplicity of syntax in mind. One of the design goals of NASM is that itshould be possible, as far as is practical, for the user to look at a single line of NASM code and tellwhat opcode is generated by it. You can’t do this in MASM: if you declare, for example,

foo equ 1 bar dw 2

then the two lines of code

mov ax,foo mov ax,bar

generate completely different opcodes, despite having identical−looking syntaxes.

NASM avoids this undesirable situation by having a much simpler syntax for memory references.The rule is simply that any access to the contents of a memory location requires square bracketsaround the address, and any access to the address of a variable doesn’t. So an instruction of theform mov ax,foo will always refer to a compile−time constant, whether it’s an EQU or theaddress of a variable; and to access the contents of the variable bar , you must codemov ax,[bar] .

This also means that NASM has no need for MASM’s OFFSET keyword, since the MASM codemov ax,offset bar means exactly the same thing as NASM’s mov ax,bar . If you’re trying

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to get large amounts of MASM code to assemble sensibly under NASM, you can always code%idefine offset to make the preprocessor treat the OFFSET keyword as a no−op.

This issue is even more confusing in a86 , where declaring a label with a trailing colon defines it tobe a ‘label’ as opposed to a ‘variable’ and causes a86 to adopt NASM−style semantics; so in a86 ,mov ax,var has different behaviour depending on whether var was declared as var: dw 0 (alabel) or var dw 0 (a word−size variable). NASM is very simple by comparison: everything is alabel.

NASM, in the interests of simplicity, also does not support the hybrid syntaxes supported byMASM and its clones, such as mov ax,table[bx] , where a memory reference is denoted byone portion outside square brackets and another portion inside. The correct syntax for the above ismov ax,[table+bx] . Likewise, mov ax,es:[di] is wrong and mov ax,[es:di] isright.

2.2.3 NASM Doesn’t Store Variable TypesNASM, by design, chooses not to remember the types of variables you declare. Whereas MASMwill remember, on seeing var dw 0 , that you declared var as a word−size variable, and will thenbe able to fill in the ambiguity in the size of the instruction mov var,2 , NASM will deliberatelyremember nothing about the symbol var except where it begins, and so you must explicitly codemov word [var],2 .

For this reason, NASM doesn’t support the LODS, MOVS, STOS, SCAS, CMPS, INS , or OUTSinstructions, but only supports the forms such as LODSB, MOVSW, and SCASD, which explicitlyspecify the size of the components of the strings being manipulated.

2.2.4 NASM Doesn’t ASSUME

As part of NASM’s drive for simplicity, it also does not support the ASSUME directive. NASM willnot keep track of what values you choose to put in your segment registers, and will neverautomatically generate a segment override prefix.

2.2.5 NASM Doesn’t Support Memory ModelsNASM also does not have any directives to support different 16−bit memory models. Theprogrammer has to keep track of which functions are supposed to be called with a far call and whichwith a near call, and is responsible for putting the correct form of RET instruction (RETN or RETF;NASM accepts RET itself as an alternate form for RETN); in addition, the programmer isresponsible for coding CALL FAR instructions where necessary when calling external functions,and must also keep track of which external variable definitions are far and which are near.

2.2.6 Floating−Point DifferencesNASM uses different names to refer to floating−point registers from MASM: where MASM wouldcall them ST(0) , ST(1) and so on, and a86 would call them simply 0, 1 and so on, NASMchooses to call them st0 , st1 etc.

As of version 0.96, NASM now treats the instructions with ‘nowait’ forms in the same way asMASM−compatible assemblers. The idiosyncratic treatment employed by 0.95 and earlier wasbased on a misunderstanding by the authors.

2.2.7 Other DifferencesFor historical reasons, NASM uses the keyword TWORD where MASM and compatible assemblersuse TBYTE.

NASM does not declare uninitialised storage in the same way as MASM: where a MASMprogrammer might use stack db 64 dup (?) , NASM requires stack resb 64 , intendedto be read as ‘reserve 64 bytes’. For a limited amount of compatibility, since NASM treats ? as a

29

valid character in symbol names, you can code ? equ 0 and then writing dw ? will at least dosomething vaguely useful. DUP is still not a supported syntax, however.

In addition to all of this, macros and directives work completely differently to MASM. See chapter4 and chapter 5 for further details.

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Chapter 3: The NASM Language

3.1 Layout of a NASM Source LineLike most assemblers, each NASM source line contains (unless it is a macro, a preprocessordirective or an assembler directive: see chapter 4 and chapter 5) some combination of the four fields

label: instruction operands ; comment

As usual, most of these fields are optional; the presence or absence of any combination of a label,an instruction and a comment is allowed. Of course, the operand field is either required or forbiddenby the presence and nature of the instruction field.

NASM uses backslash (\) as the line continuation character; if a line ends with backslash, the nextline is considered to be a part of the backslash−ended line.

NASM places no restrictions on white space within a line: labels may have white space beforethem, or instructions may have no space before them, or anything. The colon after a label is alsooptional. (Note that this means that if you intend to code lodsb alone on a line, and type lodabby accident, then that’s still a valid source line which does nothing but define a label. RunningNASM with the command−line option −w+orphan−labels will cause it to warn you if youdefine a label alone on a line without a trailing colon.)

Valid characters in labels are letters, numbers, _, $, #, @, ~, . , and ?. The only characters whichmay be used as the first character of an identifier are letters, . (with special meaning: see section3.9), _ and ?. An identifier may also be prefixed with a $ to indicate that it is intended to be read asan identifier and not a reserved word; thus, if some other module you are linking with defines asymbol called eax , you can refer to $eax in NASM code to distinguish the symbol from theregister.

The instruction field may contain any machine instruction: Pentium and P6 instructions, FPUinstructions, MMX instructions and even undocumented instructions are all supported. Theinstruction may be prefixed by LOCK, REP, REPE/REPZ or REPNE/REPNZ, in the usual way.Explicit address−size and operand−size prefixes A16, A32, O16 and O32 are provided – oneexample of their use is given in chapter 9. You can also use the name of a segment register as aninstruction prefix: coding es mov [bx],ax is equivalent to coding mov [es:bx],ax . Werecommend the latter syntax, since it is consistent with other syntactic features of the language, butfor instructions such as LODSB, which has no operands and yet can require a segment override,there is no clean syntactic way to proceed apart from es lodsb .

An instruction is not required to use a prefix: prefixes such as CS, A32, LOCK or REPE can appearon a line by themselves, and NASM will just generate the prefix bytes.

In addition to actual machine instructions, NASM also supports a number of pseudo−instructions,described in section 3.2.

Instruction operands may take a number of forms: they can be registers, described simply by theregister name (e.g. ax , bp , ebx , cr0 : NASM does not use the gas –style syntax in which registernames must be prefixed by a % sign), or they can be effective addresses (see section 3.3), constants(section 3.4) or expressions (section 3.5).

For floating−point instructions, NASM accepts a wide range of syntaxes: you can use two−operandforms like MASM supports, or you can use NASM’s native single−operand forms in most cases.

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Details of all forms of each supported instruction are given in appendix B. For example, you cancode:

fadd st1 ; this sets st0 := st0 + st1 fadd st0,st1 ; so does this

fadd st1,st0 ; this sets st1 := st1 + st0 fadd to st1 ; so does this

Almost any floating−point instruction that references memory must use one of the prefixes DWORD,QWORD or TWORD to indicate what size of memory operand it refers to.

3.2 Pseudo−InstructionsPseudo−instructions are things which, though not real x86 machine instructions, are used in theinstruction field anyway because that’s the most convenient place to put them. The currentpseudo−instructions are DB, DW, DD, DQ and DT, their uninitialised counterparts RESB, RESW,RESD, RESQ and REST, the INCBIN command, the EQU command, and the TIMES prefix.

3.2.1 DB and friends: Declaring Initialised DataDB, DW, DD, DQ and DT are used, much as in MASM, to declare initialised data in the output file.They can be invoked in a wide range of ways:

db 0x55 ; just the byte 0x55 db 0x55,0x56,0x57 ; three bytes in succession db ’a’,0x55 ; character constants are OK db ’hello’,13,10,’$’ ; so are string constants dw 0x1234 ; 0x34 0x12 dw ’a’ ; 0x41 0x00 (it’s just a number) dw ’ab’ ; 0x41 0x42 (character constant) dw ’abc’ ; 0x41 0x42 0x43 0x00 (string) dd 0x12345678 ; 0x78 0x56 0x34 0x12 dd 1.234567e20 ; floating−point constant dq 1.234567e20 ; double−precision float dt 1.234567e20 ; extended−precision float

DQ and DT do not accept numeric constants or string constants as operands.

3.2.2 RESB and friends: Declaring Uninitialised DataRESB, RESW, RESD, RESQ and REST are designed to be used in the BSS section of a module: theydeclare uninitialised storage space. Each takes a single operand, which is the number of bytes,words, doublewords or whatever to reserve. As stated in section 2.2.7, NASM does not support theMASM/TASM syntax of reserving uninitialised space by writing DW ? or similar things: this iswhat it does instead. The operand to a RESB–type pseudo−instruction is a critical expression: seesection 3.8.

For example:

buffer: resb 64 ; reserve 64 bytes wordvar: resw 1 ; reserve a word realarray resq 10 ; array of ten reals

3.2.3 INCBIN : Including External Binary FilesINCBIN is borrowed from the old Amiga assembler DevPac: it includes a binary file verbatim intothe output file. This can be handy for (for example) including graphics and sound data directly intoa game executable file. It can be called in one of these three ways:

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incbin "file.dat" ; include the whole file incbin "file.dat",1024 ; skip the first 1024 bytes incbin "file.dat",1024,512 ; skip the first 1024, and ; actually include at most 512

3.2.4 EQU: Defining ConstantsEQU defines a symbol to a given constant value: when EQU is used, the source line must contain alabel. The action of EQU is to define the given label name to the value of its (only) operand. Thisdefinition is absolute, and cannot change later. So, for example,

message db ’hello, world’ msglen equ $−message

defines msglen to be the constant 12. msglen may not then be redefined later. This is not apreprocessor definition either: the value of msglen is evaluated once, using the value of $ (seesection 3.5 for an explanation of $) at the point of definition, rather than being evaluated whereverit is referenced and using the value of $ at the point of reference. Note that the operand to an EQU isalso a critical expression (section 3.8).

3.2.5 TIMES: Repeating Instructions or DataThe TIMES prefix causes the instruction to be assembled multiple times. This is partly present asNASM’s equivalent of the DUP syntax supported by MASM–compatible assemblers, in that youcan code

zerobuf: times 64 db 0

or similar things; but TIMES is more versatile than that. The argument to TIMES is not just anumeric constant, but a numeric expression, so you can do things like

buffer: db ’hello, world’ times 64−$+buffer db ’ ’

which will store exactly enough spaces to make the total length of buffer up to 64. Finally,TIMES can be applied to ordinary instructions, so you can code trivial unrolled loops in it:

times 100 movsb

Note that there is no effective difference between times 100 resb 1 and resb 100 , exceptthat the latter will be assembled about 100 times faster due to the internal structure of the assembler.

The operand to TIMES, like that of EQU and those of RESB and friends, is a critical expression(section 3.8).

Note also that TIMES can’t be applied to macros: the reason for this is that TIMES is processedafter the macro phase, which allows the argument to TIMES to contain expressions such as64−$+buffer as above. To repeat more than one line of code, or a complex macro, use thepreprocessor %rep directive.

3.3 Effective AddressesAn effective address is any operand to an instruction which references memory. Effective addresses,in NASM, have a very simple syntax: they consist of an expression evaluating to the desiredaddress, enclosed in square brackets. For example:

wordvar dw 123 mov ax,[wordvar] mov ax,[wordvar+1] mov ax,[es:wordvar+bx]

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Anything not conforming to this simple system is not a valid memory reference in NASM, forexample es:wordvar[bx] .

More complicated effective addresses, such as those involving more than one register, work inexactly the same way:

mov eax,[ebx*2+ecx+offset] mov ax,[bp+di+8]

NASM is capable of doing algebra on these effective addresses, so that things which don’tnecessarily look legal are perfectly all right:

mov eax,[ebx*5] ; assembles as [ebx*4+ebx] mov eax,[label1*2−label2] ; ie [label1+(label1−label2)]

Some forms of effective address have more than one assembled form; in most such cases NASMwill generate the smallest form it can. For example, there are distinct assembled forms for the32−bit effective addresses [eax*2+0] and [eax+eax] , and NASM will generally generate thelatter on the grounds that the former requires four bytes to store a zero offset.

NASM has a hinting mechanism which will cause [eax+ebx] and [ebx+eax] to generatedifferent opcodes; this is occasionally useful because [esi+ebp] and [ebp+esi] have differentdefault segment registers.

However, you can force NASM to generate an effective address in a particular form by the use ofthe keywords BYTE, WORD, DWORD and NOSPLIT. If you need [eax+3] to be assembled using adouble−word offset field instead of the one byte NASM will normally generate, you can code[dword eax+3] . Similarly, you can force NASM to use a byte offset for a small value which ithasn’t seen on the first pass (see section 3.8 for an example of such a code fragment) by using[byte eax+offset] . As special cases, [byte eax] will code [eax+0] with a byte offsetof zero, and [dword eax] will code it with a double−word offset of zero. The normal form,[eax] , will be coded with no offset field.

The form described in the previous paragraph is also useful if you are trying to access data in a32−bit segment from within 16 bit code. For more information on this see the section onmixed−size addressing (section 9.2). In particular, if you need to access data with a known offsetthat is larger than will fit in a 16−bit value, if you don’t specify that it is a dword offset, nasm willcause the high word of the offset to be lost.

Similarly, NASM will split [eax*2] into [eax+eax] because that allows the offset field to beabsent and space to be saved; in fact, it will also split [eax*2+offset] into[eax+eax+offset] . You can combat this behaviour by the use of the NOSPLIT keyword:[nosplit eax*2] will force [eax*2+0] to be generated literally.

3.4 ConstantsNASM understands four different types of constant: numeric, character, string and floating−point.

3.4.1 Numeric ConstantsA numeric constant is simply a number. NASM allows you to specify numbers in a variety ofnumber bases, in a variety of ways: you can suffix H, Q and B for hex, octal and binary, or you canprefix 0x for hex in the style of C, or you can prefix $ for hex in the style of Borland Pascal. Note,though, that the $ prefix does double duty as a prefix on identifiers (see section 3.1), so a hexnumber prefixed with a $ sign must have a digit after the $ rather than a letter.

Some examples:

mov ax,100 ; decimal mov ax,0a2h ; hex

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mov ax,$0a2 ; hex again: the 0 is required mov ax,0xa2 ; hex yet again mov ax,777q ; octal mov ax,10010011b ; binary

3.4.2 Character ConstantsA character constant consists of up to four characters enclosed in either single or double quotes. Thetype of quote makes no difference to NASM, except of course that surrounding the constant withsingle quotes allows double quotes to appear within it and vice versa.

A character constant with more than one character will be arranged with little−endian order in mind:if you code

mov eax,’abcd’

then the constant generated is not 0x61626364 , but 0x64636261 , so that if you were then tostore the value into memory, it would read abcd rather than dcba . This is also the sense ofcharacter constants understood by the Pentium’s CPUID instruction (see section B.4.34).

3.4.3 String ConstantsString constants are only acceptable to some pseudo−instructions, namely the DB family andINCBIN .

A string constant looks like a character constant, only longer. It is treated as a concatenation ofmaximum−size character constants for the conditions. So the following are equivalent:

db ’hello’ ; string constant db ’h’,’e’,’l’,’l’,’o’ ; equivalent character constants

And the following are also equivalent:

dd ’ninechars’ ; doubleword string constant dd ’nine’,’char’,’s’ ; becomes three doublewords db ’ninechars’,0,0,0 ; and really looks like this

Note that when used as an operand to db , a constant like ’ab’ is treated as a string constantdespite being short enough to be a character constant, because otherwise db ’ab’ would have thesame effect as db ’a’ , which would be silly. Similarly, three−character or four−characterconstants are treated as strings when they are operands to dw.

3.4.4 Floating−Point ConstantsFloating−point constants are acceptable only as arguments to DD, DQ and DT. They are expressed inthe traditional form: digits, then a period, then optionally more digits, then optionally an E followedby an exponent. The period is mandatory, so that NASM can distinguish between dd 1 , whichdeclares an integer constant, and dd 1.0 which declares a floating−point constant.

Some examples:

dd 1.2 ; an easy one dq 1.e10 ; 10,000,000,000 dq 1.e+10 ; synonymous with 1.e10 dq 1.e−10 ; 0.000 000 000 1 dt 3.141592653589793238462 ; pi

NASM cannot do compile−time arithmetic on floating−point constants. This is because NASM isdesigned to be portable – although it always generates code to run on x86 processors, the assembleritself can run on any system with an ANSI C compiler. Therefore, the assembler cannot guaranteethe presence of a floating−point unit capable of handling the Intel number formats, and so for

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NASM to be able to do floating arithmetic it would have to include its own complete set offloating−point routines, which would significantly increase the size of the assembler for very littlebenefit.

3.5 ExpressionsExpressions in NASM are similar in syntax to those in C.

NASM does not guarantee the size of the integers used to evaluate expressions at compile time:since NASM can compile and run on 64−bit systems quite happily, don’t assume that expressionsare evaluated in 32−bit registers and so try to make deliberate use of integer overflow. It might notalways work. The only thing NASM will guarantee is what’s guaranteed by ANSI C: you alwayshave at least 32 bits to work in.

NASM supports two special tokens in expressions, allowing calculations to involve the currentassembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of theline containing the expression; so you can code an infinite loop using JMP $ . $$ evaluates to thebeginning of the current section; so you can tell how far into the section you are by using ($−$$) .

The arithmetic operators provided by NASM are listed here, in increasing order of precedence.

3.5.1 | : Bitwise OR OperatorThe | operator gives a bitwise OR, exactly as performed by the OR machine instruction. BitwiseOR is the lowest−priority arithmetic operator supported by NASM.

3.5.2 ̂ : Bitwise XOR Operator^ provides the bitwise XOR operation.

3.5.3 &: Bitwise AND Operator& provides the bitwise AND operation.

3.5.4 << and >>: Bit Shift Operators<< gives a bit−shift to the left, just as it does in C. So 5<<3 evaluates to 5 times 8, or 40. >> givesa bit−shift to the right; in NASM, such a shift is always unsigned, so that the bits shifted in from theleft−hand end are filled with zero rather than a sign−extension of the previous highest bit.

3.5.5 + and −: Addition and Subtraction OperatorsThe + and − operators do perfectly ordinary addition and subtraction.

3.5.6 * , / , // , % and %%: Multiplication and Division* is the multiplication operator. / and // are both division operators: / is unsigned division and //is signed division. Similarly, % and %% provide unsigned and signed modulo operators respectively.

NASM, like ANSI C, provides no guarantees about the sensible operation of the signed modulooperator.

Since the % character is used extensively by the macro preprocessor, you should ensure that both thesigned and unsigned modulo operators are followed by white space wherever they appear.

3.5.7 Unary Operators: +, −, ~ and SEG

The highest−priority operators in NASM’s expression grammar are those which only apply to oneargument. − negates its operand, + does nothing (it’s provided for symmetry with −), ~ computesthe one’s complement of its operand, and SEG provides the segment address of its operand(explained in more detail in section 3.6).

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3.6 SEG and WRTWhen writing large 16−bit programs, which must be split into multiple segments, it is oftennecessary to be able to refer to the segment part of the address of a symbol. NASM supports theSEG operator to perform this function.

The SEG operator returns the preferred segment base of a symbol, defined as the segment baserelative to which the offset of the symbol makes sense. So the code

mov ax,seg symbol mov es,ax mov bx,symbol

will load ES:BX with a valid pointer to the symbol symbol .

Things can be more complex than this: since 16−bit segments and groups may overlap, you mightoccasionally want to refer to some symbol using a different segment base from the preferred one.NASM lets you do this, by the use of the WRT (With Reference To) keyword. So you can do thingslike

mov ax,weird_seg ; weird_seg is a segment base mov es,ax mov bx,symbol wrt weird_seg

to load ES:BX with a different, but functionally equivalent, pointer to the symbol symbol .

NASM supports far (inter−segment) calls and jumps by means of the syntaxcall segment:offset , where segment and offset both represent immediate values. So tocall a far procedure, you could code either of

call (seg procedure):procedure call weird_seg:(procedure wrt weird_seg)

(The parentheses are included for clarity, to show the intended parsing of the above instructions.They are not necessary in practice.)

NASM supports the syntax call far procedure as a synonym for the first of the aboveusages. JMP works identically to CALL in these examples.

To declare a far pointer to a data item in a data segment, you must code

dw symbol, seg symbol

NASM supports no convenient synonym for this, though you can always invent one using themacro processor.

3.7 STRICT: Inhibiting OptimizationWhen assembling with the optimizer set to level 2 or higher (see section 2.1.15), NASM will useesize specifiers (BYTE, WORD, DWORD, QWORD, or TWORD), but will give them the smallest possiblesize. The keyword STRICT can be used to inhibit optimization and force a particular operand to beemitted in the specified size. For example, with the optimizer on, and in BITS 16 mode,

push dword 33

is encoded in three bytes 66 6A 21 , whereas

push strict dword 33

is encoded in six bytes, with a full dword immediate operand 66 68 21 00 00 00 .

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With the optimizer off, the same code (six bytes) is generated whether the STRICT keyword wasused or not.

3.8 Critical ExpressionsA limitation of NASM is that it is a two−pass assembler; unlike TASM and others, it will always doexactly two assembly passes. Therefore it is unable to cope with source files that are complexenough to require three or more passes.

The first pass is used to determine the size of all the assembled code and data, so that the secondpass, when generating all the code, knows all the symbol addresses the code refers to. So one thingNASM can’t handle is code whose size depends on the value of a symbol declared after the code inquestion. For example,

times (label−$) db 0 label: db ’Where am I?’

The argument to TIMES in this case could equally legally evaluate to anything at all; NASM willreject this example because it cannot tell the size of the TIMES line when it first sees it. It will justas firmly reject the slightly paradoxical code

times (label−$+1) db 0 label: db ’NOW where am I?’

in which any value for the TIMES argument is by definition wrong!

NASM rejects these examples by means of a concept called a critical expression, which is definedto be an expression whose value is required to be computable in the first pass, and which musttherefore depend only on symbols defined before it. The argument to the TIMES prefix is a criticalexpression; for the same reason, the arguments to the RESB family of pseudo−instructions are alsocritical expressions.

Critical expressions can crop up in other contexts as well: consider the following code.

mov ax,symbol1 symbol1 equ symbol2 symbol2:

On the first pass, NASM cannot determine the value of symbol1 , because symbol1 is defined tobe equal to symbol2 which NASM hasn’t seen yet. On the second pass, therefore, when itencounters the line mov ax,symbol1 , it is unable to generate the code for it because it stilldoesn’t know the value of symbol1 . On the next line, it would see the EQU again and be able todetermine the value of symbol1 , but by then it would be too late.

NASM avoids this problem by defining the right−hand side of an EQU statement to be a criticalexpression, so the definition of symbol1 would be rejected in the first pass.

There is a related issue involving forward references: consider this code fragment.

mov eax,[ebx+offset] offset equ 10

NASM, on pass one, must calculate the size of the instruction mov eax,[ebx+offset]without knowing the value of offset . It has no way of knowing that offset is small enough tofit into a one−byte offset field and that it could therefore get away with generating a shorter form ofthe effective−address encoding; for all it knows, in pass one, offset could be a symbol in thecode segment, and it might need the full four−byte form. So it is forced to compute the size of theinstruction to accommodate a four−byte address part. In pass two, having made this decision, it isnow forced to honour it and keep the instruction large, so the code generated in this case is not as

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small as it could have been. This problem can be solved by defining offset before using it, or byforcing byte size in the effective address by coding [byte ebx+offset] .

3.9 Local LabelsNASM gives special treatment to symbols beginning with a period. A label beginning with a singleperiod is treated as a local label, which means that it is associated with the previous non−local label.So, for example:

label1 ; some code

.loop ; some more code

jne .loop ret

label2 ; some code

.loop ; some more code

jne .loop ret

In the above code fragment, each JNE instruction jumps to the line immediately before it, becausethe two definitions of .loop are kept separate by virtue of each being associated with the previousnon−local label.

This form of local label handling is borrowed from the old Amiga assembler DevPac; however,NASM goes one step further, in allowing access to local labels from other parts of the code. This isachieved by means of defining a local label in terms of the previous non−local label: the firstdefinition of .loop above is really defining a symbol called label1.loop , and the seconddefines a symbol called label2.loop . So, if you really needed to, you could write

label3 ; some more code ; and some more

jmp label1.loop

Sometimes it is useful – in a macro, for instance – to be able to define a label which can bereferenced from anywhere but which doesn’t interfere with the normal local−label mechanism. Sucha label can’t be non−local because it would interfere with subsequent definitions of, and referencesto, local labels; and it can’t be local because the macro that defined it wouldn’t know the label’s fullname. NASM therefore introduces a third type of label, which is probably only useful in macrodefinitions: if a label begins with the special prefix ..@ , then it does nothing to the local labelmechanism. So you could code

label1: ; a non−local label .local: ; this is really label1.local ..@@foo: ; this is a special symbol label2: ; another non−local label .local: ; this is really label2.local

jmp ..@@foo ; this will jump three lines up

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NASM has the capacity to define other special symbols beginning with a double period: forexample, ..start is used to specify the entry point in the obj output format (see section 6.2.6).

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Chapter 4: The NASM Preprocessor

NASM contains a powerful macro processor, which supports conditional assembly, multi−level fileinclusion, two forms of macro (single−line and multi−line), and a ‘context stack’ mechanism forextra macro power. Preprocessor directives all begin with a % sign.

The preprocessor collapses all lines which end with a backslash (\) character into a single line. Thus:

%define THIS_VERY_LONG_MACRO_NAME_IS_DEFINED_TO \\ THIS_VALUE

will work like a single−line macro without the backslash−newline sequence.

4.1 Single−Line Macros4.1.1 The Normal Way: %define

Single−line macros are defined using the %define preprocessor directive. The definitions work ina similar way to C; so you can do things like

%define ctrl 0x1F & %define param(a,b) ((a)+(a)*(b))

mov byte [param(2,ebx)], ctrl ’D’

which will expand to

mov byte [(2)+(2)*(ebx)], 0x1F & ’D’

When the expansion of a single−line macro contains tokens which invoke another macro, theexpansion is performed at invocation time, not at definition time. Thus the code

%define a(x) 1+b(x) %define b(x) 2*x

mov ax,a(8)

will evaluate in the expected way to mov ax,1+2*8 , even though the macro b wasn’t defined atthe time of definition of a.

Macros defined with %define are case sensitive: after %define foo bar , only foo willexpand to bar : Foo or FOO will not. By using %idefine instead of %define (the ‘i’ stands for‘insensitive’) you can define all the case variants of a macro at once, so that %idefine foo barwould cause foo , Foo, FOO, fOO and so on all to expand to bar .

There is a mechanism which detects when a macro call has occurred as a result of a previousexpansion of the same macro, to guard against circular references and infinite loops. If this happens,the preprocessor will only expand the first occurrence of the macro. Hence, if you code

%define a(x) 1+a(x)

mov ax,a(3)

the macro a(3) will expand once, becoming 1+a(3) , and will then expand no further. Thisbehaviour can be useful: see section 8.1 for an example of its use.

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You can overload single−line macros: if you write

%define foo(x) 1+x %define foo(x,y) 1+x*y

the preprocessor will be able to handle both types of macro call, by counting the parameters youpass; so foo(3) will become 1+3 whereas foo(ebx,2) will become 1+ebx*2 . However, ifyou define

%define foo bar

then no other definition of foo will be accepted: a macro with no parameters prohibits thedefinition of the same name as a macro with parameters, and vice versa.

This doesn’t prevent single−line macros being redefined: you can perfectly well define a macro with

%define foo bar

and then re−define it later in the same source file with

%define foo baz

Then everywhere the macro foo is invoked, it will be expanded according to the most recentdefinition. This is particularly useful when defining single−line macros with %assign (see section4.1.5).

You can pre−define single−line macros using the ‘−d’ option on the NASM command line: seesection 2.1.11.

4.1.2 Enhancing %define: %xdefine

To have a reference to an embedded single−line macro resolved at the time that it is embedded, asopposed to when the calling macro is expanded, you need a different mechanism to the one offeredby %define . The solution is to use %xdefine , or it’s case−insensitive counterpart %xidefine .

Suppose you have the following code:

%define isTrue 1 %define isFalse isTrue %define isTrue 0

val1: db isFalse

%define isTrue 1

val2: db isFalse

In this case, val1 is equal to 0, and val2 is equal to 1. This is because, when a single−line macrois defined using %define , it is expanded only when it is called. As isFalse expands toisTrue , the expansion will be the current value of isTrue . The first time it is called that is 0, andthe second time it is 1.

If you wanted isFalse to expand to the value assigned to the embedded macro isTrue at thetime that isFalse was defined, you need to change the above code to use %xdefine .

%xdefine isTrue 1 %xdefine isFalse isTrue %xdefine isTrue 0

val1: db isFalse

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%xdefine isTrue 1

val2: db isFalse

Now, each time that isFalse is called, it expands to 1, as that is what the embedded macroisTrue expanded to at the time that isFalse was defined.

4.1.3 Concatenating Single Line Macro Tokens: %+

Individual tokens in single line macros can be concatenated, to produce longer tokens for laterprocessing. This can be useful if there are several similar macros that perform similar functions.

As an example, consider the following:

%define BDASTART 400h ; Start of BIOS data area

struc tBIOSDA ; its structure .COM1addr RESW 1 .COM2addr RESW 1 ; ..and so on endstruc

Now, if we need to access the elements of tBIOSDA in different places, we can end up with:

mov ax,BDASTART + tBIOSDA.COM1addr mov bx,BDASTART + tBIOSDA.COM2addr

This will become pretty ugly (and tedious) if used in many places, and can be reduced in sizesignificantly by using the following macro:

; Macro to access BIOS variables by their names (from tBDA):

%define BDA(x) BDASTART + tBIOSDA. %+ x

Now the above code can be written as:

mov ax,BDA(COM1addr) mov bx,BDA(COM2addr)

Using this feature, we can simplify references to a lot of macros (and, in turn, reduce typing errors).

4.1.4 Undefining macros: %undef

Single−line macros can be removed with the %undef command. For example, the followingsequence:

%define foo bar %undef foo

mov eax, foo

will expand to the instruction mov eax, foo , since after %undef the macro foo is no longerdefined.

Macros that would otherwise be pre−defined can be undefined on the command−line using the ‘−u’option on the NASM command line: see section 2.1.12.

4.1.5 Preprocessor Variables: %assign

An alternative way to define single−line macros is by means of the %assign command (and itscase−insensitive counterpart %iassign , which differs from %assign in exactly the same waythat %idefine differs from %define ).

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%assign is used to define single−line macros which take no parameters and have a numeric value.This value can be specified in the form of an expression, and it will be evaluated once, when the%assign directive is processed.

Like %define , macros defined using %assign can be re−defined later, so you can do things like

%assign i i+1

to increment the numeric value of a macro.

%assign is useful for controlling the termination of %rep preprocessor loops: see section 4.5 foran example of this. Another use for %assign is given in section 7.4 and section 8.1.

The expression passed to %assign is a critical expression (see section 3.8), and must also evaluateto a pure number (rather than a relocatable reference such as a code or data address, or anythinginvolving a register).

4.2 String Handling in Macros: %strlen and %substrIt’s often useful to be able to handle strings in macros. NASM supports two simple string handlingmacro operators from which more complex operations can be constructed.

4.2.1 String Length: %strlen

The %strlen macro is like %assign macro in that it creates (or redefines) a numeric value to amacro. The difference is that with %strlen , the numeric value is the length of a string. Anexample of the use of this would be:

%strlen charcnt ’my string’

In this example, charcnt would receive the value 8, just as if an %assign had been used. In thisexample, ’my string’ was a literal string but it could also have been a single−line macro thatexpands to a string, as in the following example:

%define sometext ’my string’ %strlen charcnt sometext

As in the first case, this would result in charcnt being assigned the value of 8.

4.2.2 Sub−strings: %substr

Individual letters in strings can be extracted using %substr . An example of its use is probablymore useful than the description:

%substr mychar ’xyz’ 1 ; equivalent to %define mychar ’x’ %substr mychar ’xyz’ 2 ; equivalent to %define mychar ’y’ %substr mychar ’xyz’ 3 ; equivalent to %define mychar ’z’

In this example, mychar gets the value of ’y’. As with %strlen (see section 4.2.1), the firstparameter is the single−line macro to be created and the second is the string. The third parameterspecifies which character is to be selected. Note that the first index is 1, not 0 and the last index isequal to the value that %strlen would assign given the same string. Index values out of rangeresult in an empty string.

4.3 Multi−Line Macros: %macroMulti−line macros are much more like the type of macro seen in MASM and TASM: a multi−linemacro definition in NASM looks something like this.

%macro prologue 1

push ebp

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mov ebp,esp sub esp,%1

%endmacro

This defines a C−like function prologue as a macro: so you would invoke the macro with a call suchas

myfunc: prologue 12

which would expand to the three lines of code

myfunc: push ebp mov ebp,esp sub esp,12

The number 1 after the macro name in the %macro line defines the number of parameters themacro prologue expects to receive. The use of %1 inside the macro definition refers to the firstparameter to the macro call. With a macro taking more than one parameter, subsequent parameterswould be referred to as %2, %3 and so on.

Multi−line macros, like single−line macros, are case−sensitive, unless you define them using thealternative directive %imacro .

If you need to pass a comma as part of a parameter to a multi−line macro, you can do that byenclosing the entire parameter in braces. So you could code things like

%macro silly 2

%2: db %1

%endmacro

silly ’a’, letter_a ; letter_a: db ’a’ silly ’ab’, string_ab ; string_ab: db ’ab’ silly @\{13,10@\}, crlf ; crlf: db 13,10

4.3.1 Overloading Multi−Line MacrosAs with single−line macros, multi−line macros can be overloaded by defining the same macro nameseveral times with different numbers of parameters. This time, no exception is made for macroswith no parameters at all. So you could define

%macro prologue 0

push ebp mov ebp,esp

%endmacro

to define an alternative form of the function prologue which allocates no local stack space.

Sometimes, however, you might want to ‘overload’ a machine instruction; for example, you mightwant to define

%macro push 2

push %1 push %2

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%endmacro

so that you could code

push ebx ; this line is not a macro call push eax,ecx ; but this one is

Ordinarily, NASM will give a warning for the first of the above two lines, since push is nowdefined to be a macro, and is being invoked with a number of parameters for which no definitionhas been given. The correct code will still be generated, but the assembler will give a warning. Thiswarning can be disabled by the use of the −w−macro−params command−line option (see section2.1.17).

4.3.2 Macro−Local LabelsNASM allows you to define labels within a multi−line macro definition in such a way as to makethem local to the macro call: so calling the same macro multiple times will use a different label eachtime. You do this by prefixing %% to the label name. So you can invent an instruction whichexecutes a RET if the Z flag is set by doing this:

%macro retz 0

jnz %%skip ret %%skip:

%endmacro

You can call this macro as many times as you want, and every time you call it NASM will make upa different ‘real’ name to substitute for the label %%skip. The names NASM invents are of theform [email protected] , where the number 2345 changes with every macro call. The ..@ prefixprevents macro−local labels from interfering with the local label mechanism, as described in section3.9. You should avoid defining your own labels in this form (the ..@ prefix, then a number, thenanother period) in case they interfere with macro−local labels.

4.3.3 Greedy Macro ParametersOccasionally it is useful to define a macro which lumps its entire command line into one parameterdefinition, possibly after extracting one or two smaller parameters from the front. An examplemight be a macro to write a text string to a file in MS−DOS, where you might want to be able towrite

writefile [filehandle],"hello, world",13,10

NASM allows you to define the last parameter of a macro to be greedy, meaning that if you invokethe macro with more parameters than it expects, all the spare parameters get lumped into the lastdefined one along with the separating commas. So if you code:

%macro writefile 2+

jmp %%endstr %%str: db %2 %%endstr: mov dx,%%str mov cx,%%endstr−%%str mov bx,%1 mov ah,0x40 int 0x21

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%endmacro

then the example call to writefile above will work as expected: the text before the first comma,[filehandle] , is used as the first macro parameter and expanded when %1 is referred to, and allthe subsequent text is lumped into %2 and placed after the db .

The greedy nature of the macro is indicated to NASM by the use of the + sign after the parametercount on the %macro line.

If you define a greedy macro, you are effectively telling NASM how it should expand the macrogiven any number of parameters from the actual number specified up to infinity; in this case, forexample, NASM now knows what to do when it sees a call to writefile with 2, 3, 4 or moreparameters. NASM will take this into account when overloading macros, and will not allow you todefine another form of writefile taking 4 parameters (for example).

Of course, the above macro could have been implemented as a non−greedy macro, in which casethe call to it would have had to look like

writefile [filehandle], @\{"hello, world",13,10@\}

NASM provides both mechanisms for putting commas in macro parameters, and you choose whichone you prefer for each macro definition.

See section 5.2.1 for a better way to write the above macro.

4.3.4 Default Macro ParametersNASM also allows you to define a multi−line macro with a range of allowable parameter counts. Ifyou do this, you can specify defaults for omitted parameters. So, for example:

%macro die 0−1 "Painful program death has occurred."

writefile 2,%1 mov ax,0x4c01 int 0x21

%endmacro

This macro (which makes use of the writefile macro defined in section 4.3.3) can be calledwith an explicit error message, which it will display on the error output stream before exiting, or itcan be called with no parameters, in which case it will use the default error message supplied in themacro definition.

In general, you supply a minimum and maximum number of parameters for a macro of this type; theminimum number of parameters are then required in the macro call, and then you provide defaultsfor the optional ones. So if a macro definition began with the line

%macro foobar 1−3 eax,[ebx+2]

then it could be called with between one and three parameters, and %1 would always be taken fromthe macro call. %2, if not specified by the macro call, would default to eax , and %3 if not specifiedwould default to [ebx+2] .

You may omit parameter defaults from the macro definition, in which case the parameter default istaken to be blank. This can be useful for macros which can take a variable number of parameters,since the %0 token (see section 4.3.5) allows you to determine how many parameters were reallypassed to the macro call.

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This defaulting mechanism can be combined with the greedy−parameter mechanism; so the diemacro above could be made more powerful, and more useful, by changing the first line of thedefinition to

%macro die 0−1+ "Painful program death has occurred.",13,10

The maximum parameter count can be infinite, denoted by * . In this case, of course, it is impossibleto provide a full set of default parameters. Examples of this usage are shown in section 4.3.6.

4.3.5 %0: Macro Parameter CounterFor a macro which can take a variable number of parameters, the parameter reference %0 will returna numeric constant giving the number of parameters passed to the macro. This can be used as anargument to %rep (see section 4.5) in order to iterate through all the parameters of a macro.Examples are given in section 4.3.6.

4.3.6 %rotate : Rotating Macro ParametersUnix shell programmers will be familiar with the shift shell command, which allows thearguments passed to a shell script (referenced as $1 , $2 and so on) to be moved left by one place,so that the argument previously referenced as $2 becomes available as $1 , and the argumentpreviously referenced as $1 is no longer available at all.

NASM provides a similar mechanism, in the form of %rotate . As its name suggests, it differsfrom the Unix shift in that no parameters are lost: parameters rotated off the left end of theargument list reappear on the right, and vice versa.

%rotate is invoked with a single numeric argument (which may be an expression). The macroparameters are rotated to the left by that many places. If the argument to %rotate is negative, themacro parameters are rotated to the right.

So a pair of macros to save and restore a set of registers might work as follows:

%macro multipush 1−*

%rep %0 push %1 %rotate 1 %endrep

%endmacro

This macro invokes the PUSH instruction on each of its arguments in turn, from left to right. Itbegins by pushing its first argument, %1, then invokes %rotate to move all the arguments oneplace to the left, so that the original second argument is now available as %1. Repeating thisprocedure as many times as there were arguments (achieved by supplying %0 as the argument to%rep) causes each argument in turn to be pushed.

Note also the use of * as the maximum parameter count, indicating that there is no upper limit onthe number of parameters you may supply to the multipush macro.

It would be convenient, when using this macro, to have a POP equivalent, which didn’t require thearguments to be given in reverse order. Ideally, you would write the multipush macro call, thencut−and−paste the line to where the pop needed to be done, and change the name of the calledmacro to multipop , and the macro would take care of popping the registers in the opposite orderfrom the one in which they were pushed.

This can be done by the following definition:

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%macro multipop 1−*

%rep %0 %rotate −1 pop %1 %endrep

%endmacro

This macro begins by rotating its arguments one place to the right, so that the original last argumentappears as %1. This is then popped, and the arguments are rotated right again, so the second−to−lastargument becomes %1. Thus the arguments are iterated through in reverse order.

4.3.7 Concatenating Macro ParametersNASM can concatenate macro parameters on to other text surrounding them. This allows you todeclare a family of symbols, for example, in a macro definition. If, for example, you wanted togenerate a table of key codes along with offsets into the table, you could code something like

%macro keytab_entry 2

keypos%1 equ $−keytab db %2

%endmacro

keytab: keytab_entry F1,128+1 keytab_entry F2,128+2 keytab_entry Return,13

which would expand to

keytab: keyposF1 equ $−keytab db 128+1 keyposF2 equ $−keytab db 128+2 keyposReturn equ $−keytab db 13

You can just as easily concatenate text on to the other end of a macro parameter, by writing %1foo .

If you need to append a digit to a macro parameter, for example defining labels foo1 and foo2when passed the parameter foo , you can’t code %11 because that would be taken as the eleventhmacro parameter. Instead, you must code %{1}1 , which will separate the first 1 (giving the numberof the macro parameter) from the second (literal text to be concatenated to the parameter).

This concatenation can also be applied to other preprocessor in−line objects, such as macro−locallabels (section 4.3.2) and context−local labels (section 4.7.2). In all cases, ambiguities in syntax canbe resolved by enclosing everything after the % sign and before the literal text in braces: so%{%foo}bar concatenates the text bar to the end of the real name of the macro−local label%%foo. (This is unnecessary, since the form NASM uses for the real names of macro−local labelsmeans that the two usages %{%foo}bar and %%foobar would both expand to the same thinganyway; nevertheless, the capability is there.)

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4.3.8 Condition Codes as Macro ParametersNASM can give special treatment to a macro parameter which contains a condition code. For astart, you can refer to the macro parameter %1 by means of the alternative syntax %+1, whichinforms NASM that this macro parameter is supposed to contain a condition code, and will causethe preprocessor to report an error message if the macro is called with a parameter which is not avalid condition code.

Far more usefully, though, you can refer to the macro parameter by means of %−1, which NASMwill expand as the inverse condition code. So the retz macro defined in section 4.3.2 can bereplaced by a general conditional−return macro like this:

%macro retc 1

j%−1 %%skip ret %%skip:

%endmacro

This macro can now be invoked using calls like retc ne , which will cause the conditional−jumpinstruction in the macro expansion to come out as JE, or retc po which will make the jump aJPE.

The %+1 macro−parameter reference is quite happy to interpret the arguments CXZ and ECXZ asvalid condition codes; however, %−1 will report an error if passed either of these, because noinverse condition code exists.

4.3.9 Disabling Listing ExpansionWhen NASM is generating a listing file from your program, it will generally expand multi−linemacros by means of writing the macro call and then listing each line of the expansion. This allowsyou to see which instructions in the macro expansion are generating what code; however, for somemacros this clutters the listing up unnecessarily.

NASM therefore provides the .nolist qualifier, which you can include in a macro definition toinhibit the expansion of the macro in the listing file. The .nolist qualifier comes directly afterthe number of parameters, like this:

%macro foo 1.nolist

Or like this:

%macro bar 1−5+.nolist a,b,c,d,e,f,g,h

4.4 Conditional AssemblySimilarly to the C preprocessor, NASM allows sections of a source file to be assembled only ifcertain conditions are met. The general syntax of this feature looks like this:

%if<condition> ; some code which only appears if <condition> is met %elif<condition2> ; only appears if <condition> is not met but <condition2> is %else ; this appears if neither <condition> nor <condition2> was met %endif

The %else clause is optional, as is the %elif clause. You can have more than one %elif clauseas well.

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4.4.1 %ifdef : Testing Single−Line Macro ExistenceBeginning a conditional−assembly block with the line %ifdef MACRO will assemble thesubsequent code if, and only if, a single−line macro called MACRO is defined. If not, then the%elif and %else blocks (if any) will be processed instead.

For example, when debugging a program, you might want to write code such as

; perform some function %ifdef DEBUG writefile 2,"Function performed successfully",13,10 %endif ; go and do something else

Then you could use the command−line option −dDEBUG to create a version of the program whichproduced debugging messages, and remove the option to generate the final release version of theprogram.

You can test for a macro not being defined by using %ifndef instead of %ifdef . You can alsotest for macro definitions in %elif blocks by using %elifdef and %elifndef .

4.4.2 ifmacro : Testing Multi−Line Macro ExistenceThe %ifmacro directive operates in the same way as the %ifdef directive, except that it checksfor the existence of a multi−line macro.

For example, you may be working with a large project and not have control over the macros in alibrary. You may want to create a macro with one name if it doesn’t already exist, and another nameif one with that name does exist.

The %ifmacro is considered true if defining a macro with the given name and number ofarguments would cause a definitions conflict. For example:

%ifmacro MyMacro 1−3

%error "MyMacro 1−3" causes a conflict with an existing macro.

%else

%macro MyMacro 1−3

; insert code to define the macro

%endmacro

%endif

This will create the macro "MyMacro 1−3" if no macro already exists which would conflict with it,and emits a warning if there would be a definition conflict.

You can test for the macro not existing by using the %ifnmacro instead of %ifmacro .Additional tests can be performed in %elif blocks by using %elifmacro and %elifnmacro .

4.4.3 %ifctx : Testing the Context StackThe conditional−assembly construct %ifctx ctxname will cause the subsequent code to beassembled if and only if the top context on the preprocessor’s context stack has the namectxname . As with %ifdef , the inverse and %elif forms %ifnctx , %elifctx and%elifnctx are also supported.

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For more details of the context stack, see section 4.7. For a sample use of %ifctx , see section4.7.5.

4.4.4 %if : Testing Arbitrary Numeric ExpressionsThe conditional−assembly construct %if expr will cause the subsequent code to be assembled ifand only if the value of the numeric expression expr is non−zero. An example of the use of thisfeature is in deciding when to break out of a %rep preprocessor loop: see section 4.5 for a detailedexample.

The expression given to %if , and its counterpart %elif , is a critical expression (see section 3.8).

%if extends the normal NASM expression syntax, by providing a set of relational operators whichare not normally available in expressions. The operators =, <, >, <=, >= and <> test equality,less−than, greater−than, less−or−equal, greater−or−equal and not−equal respectively. The C−likeforms == and != are supported as alternative forms of = and <>. In addition, low−priority logicaloperators &&, ^^ and || are provided, supplying logical AND, logical XOR and logical OR. Thesework like the C logical operators (although C has no logical XOR), in that they always return either0 or 1, and treat any non−zero input as 1 (so that ^^ , for example, returns 1 if exactly one of itsinputs is zero, and 0 otherwise). The relational operators also return 1 for true and 0 for false.

4.4.5 %ifidn and %ifidni : Testing Exact Text IdentityThe construct %ifidn text1,text2 will cause the subsequent code to be assembled if andonly if text1 and text2 , after expanding single−line macros, are identical pieces of text.Differences in white space are not counted.

%ifidni is similar to %ifidn , but is case−insensitive.

For example, the following macro pushes a register or number on the stack, and allows you to treatIP as a real register:

%macro pushparam 1

%ifidni %1,ip call %%label %%label: %else push %1 %endif

%endmacro

Like most other %if constructs, %ifidn has a counterpart %elifidn , and negative forms%ifnidn and %elifnidn . Similarly, %ifidni has counterparts %elifidni , %ifnidni and%elifnidni .

4.4.6 %ifid , %ifnum , %ifstr : Testing Token TypesSome macros will want to perform different tasks depending on whether they are passed a number,a string, or an identifier. For example, a string output macro might want to be able to cope withbeing passed either a string constant or a pointer to an existing string.

The conditional assembly construct %ifid , taking one parameter (which may be blank), assemblesthe subsequent code if and only if the first token in the parameter exists and is an identifier.%ifnum works similarly, but tests for the token being a numeric constant; %ifstr tests for itbeing a string.

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For example, the writefile macro defined in section 4.3.3 can be extended to take advantage of%ifstr in the following fashion:

%macro writefile 2−3+

%ifstr %2 jmp %%endstr %if %0 = 3 %%str: db %2,%3 %else %%str: db %2 %endif %%endstr: mov dx,%%str mov cx,%%endstr−%%str %else mov dx,%2 mov cx,%3 %endif mov bx,%1 mov ah,0x40 int 0x21

%endmacro

Then the writefile macro can cope with being called in either of the following two ways:

writefile [file], strpointer, length writefile [file], "hello", 13, 10

In the first, strpointer is used as the address of an already−declared string, and length isused as its length; in the second, a string is given to the macro, which therefore declares it itself andworks out the address and length for itself.

Note the use of %if inside the %ifstr : this is to detect whether the macro was passed twoarguments (so the string would be a single string constant, and db %2 would be adequate) or more(in which case, all but the first two would be lumped together into %3, and db %2,%3 would berequired).

The usual %elifXXX , %ifnXXX and %elifnXXX versions exist for each of %ifid , %ifnumand %ifstr .

4.4.7 %error : Reporting User−Defined ErrorsThe preprocessor directive %error will cause NASM to report an error if it occurs in assembledcode. So if other users are going to try to assemble your source files, you can ensure that they definethe right macros by means of code like this:

%ifdef SOME_MACRO ; do some setup %elifdef SOME_OTHER_MACRO ; do some different setup %else %error Neither SOME_MACRO nor SOME_OTHER_MACRO was defined. %endif

Then any user who fails to understand the way your code is supposed to be assembled will bequickly warned of their mistake, rather than having to wait until the program crashes on being runand then not knowing what went wrong.

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4.5 Preprocessor Loops: %repNASM’s TIMES prefix, though useful, cannot be used to invoke a multi−line macro multiple times,because it is processed by NASM after macros have already been expanded. Therefore NASMprovides another form of loop, this time at the preprocessor level: %rep.

The directives %rep and %endrep (%rep takes a numeric argument, which can be an expression;%endrep takes no arguments) can be used to enclose a chunk of code, which is then replicated asmany times as specified by the preprocessor:

%assign i 0 %rep 64 inc word [table+2*i] %assign i i+1 %endrep

This will generate a sequence of 64 INC instructions, incrementing every word of memory from[table] to [table+126] .

For more complex termination conditions, or to break out of a repeat loop part way along, you canuse the %exitrep directive to terminate the loop, like this:

fibonacci: %assign i 0 %assign j 1 %rep 100 %if j > 65535 %exitrep %endif dw j %assign k j+i %assign i j %assign j k %endrep

fib_number equ ($−fibonacci)/2

This produces a list of all the Fibonacci numbers that will fit in 16 bits. Note that a maximum repeatcount must still be given to %rep. This is to prevent the possibility of NASM getting into aninfinite loop in the preprocessor, which (on multitasking or multi−user systems) would typicallycause all the system memory to be gradually used up and other applications to start crashing.

4.6 Including Other FilesUsing, once again, a very similar syntax to the C preprocessor, NASM’s preprocessor lets youinclude other source files into your code. This is done by the use of the %include directive:

%include "macros.mac"

will include the contents of the file macros.mac into the source file containing the %includedirective.

Include files are searched for in the current directory (the directory you’re in when you run NASM,as opposed to the location of the NASM executable or the location of the source file), plus anydirectories specified on the NASM command line using the −i option.

The standard C idiom for preventing a file being included more than once is just as applicable inNASM: if the file macros.mac has the form

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%ifndef MACROS_MAC %define MACROS_MAC ; now define some macros %endif

then including the file more than once will not cause errors, because the second time the file isincluded nothing will happen because the macro MACROS_MAC will already be defined.

You can force a file to be included even if there is no %include directive that explicitly includesit, by using the −p option on the NASM command line (see section 2.1.10).

4.7 The Context StackHaving labels that are local to a macro definition is sometimes not quite powerful enough:sometimes you want to be able to share labels between several macro calls. An example might be aREPEAT ... UNTIL loop, in which the expansion of the REPEAT macro would need to be able torefer to a label which the UNTIL macro had defined. However, for such a macro you would alsowant to be able to nest these loops.

NASM provides this level of power by means of a context stack. The preprocessor maintains a stackof contexts, each of which is characterised by a name. You add a new context to the stack using the%push directive, and remove one using %pop. You can define labels that are local to a particularcontext on the stack.

4.7.1 %push and %pop: Creating and Removing ContextsThe %push directive is used to create a new context and place it on the top of the context stack.%push requires one argument, which is the name of the context. For example:

%push foobar

This pushes a new context called foobar on the stack. You can have several contexts on the stackwith the same name: they can still be distinguished.

The directive %pop, requiring no arguments, removes the top context from the context stack anddestroys it, along with any labels associated with it.

4.7.2 Context−Local LabelsJust as the usage %%foo defines a label which is local to the particular macro call in which it isused, the usage %$foo is used to define a label which is local to the context on the top of thecontext stack. So the REPEAT and UNTIL example given above could be implemented by means of:

%macro repeat 0

%push repeat %$begin:

%endmacro

%macro until 1

j%−1 %$begin %pop

%endmacro

and invoked by means of, for example,

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mov cx,string repeat add cx,3 scasb until e

which would scan every fourth byte of a string in search of the byte in AL.

If you need to define, or access, labels local to the context below the top one on the stack, you canuse %$$foo , or %$$$foo for the context below that, and so on.

4.7.3 Context−Local Single−Line MacrosNASM also allows you to define single−line macros which are local to a particular context, in justthe same way:

%define %$localmac 3

will define the single−line macro %$localmac to be local to the top context on the stack. Ofcourse, after a subsequent %push, it can then still be accessed by the name %$$localmac .

4.7.4 %repl : Renaming a ContextIf you need to change the name of the top context on the stack (in order, for example, to have itrespond differently to %ifctx ), you can execute a %pop followed by a %push; but this will havethe side effect of destroying all context−local labels and macros associated with the context that wasjust popped.

NASM provides the directive %repl , which replaces a context with a different name, withouttouching the associated macros and labels. So you could replace the destructive code

%pop %push newname

with the non−destructive version %repl newname .

4.7.5 Example Use of the Context Stack: Block IFsThis example makes use of almost all the context−stack features, including theconditional−assembly construct %ifctx , to implement a block IF statement as a set of macros.

%macro if 1

%push if j%−1 %$ifnot

%endmacro

%macro else 0

%ifctx if %repl else jmp %$ifend %$ifnot: %else %error "expected ‘if’ before ‘else’" %endif

%endmacro

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%macro endif 0

%ifctx if %$ifnot: %pop %elifctx else %$ifend: %pop %else %error "expected ‘if’ or ‘else’ before ‘endif’" %endif

%endmacro

This code is more robust than the REPEAT and UNTIL macros given in section 4.7.2, because ituses conditional assembly to check that the macros are issued in the right order (for example, notcalling endif before if ) and issues a %error if they’re not.

In addition, the endif macro has to be able to cope with the two distinct cases of either directlyfollowing an if , or following an else . It achieves this, again, by using conditional assembly to dodifferent things depending on whether the context on top of the stack is if or else .

The else macro has to preserve the context on the stack, in order to have the %$ifnot referred toby the if macro be the same as the one defined by the endif macro, but has to change thecontext’s name so that endif will know there was an intervening else . It does this by the use of%repl .

A sample usage of these macros might look like:

cmp ax,bx

if ae cmp bx,cx

if ae mov ax,cx else mov ax,bx endif

else cmp ax,cx

if ae mov ax,cx endif

endif

The block−IF macros handle nesting quite happily, by means of pushing another context,describing the inner if , on top of the one describing the outer if ; thus else and endif alwaysrefer to the last unmatched if or else .

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4.8 Standard MacrosNASM defines a set of standard macros, which are already defined when it starts to process anysource file. If you really need a program to be assembled with no pre−defined macros, you can usethe %clear directive to empty the preprocessor of everything.

Most user−level assembler directives (see chapter 5) are implemented as macros which invokeprimitive directives; these are described in chapter 5. The rest of the standard macro set is describedhere.

4.8.1 __NASM_MAJOR__, __NASM_MINOR__, __NASM_SUBMINOR__ and___NASM_PATCHLEVEL__: NASM VersionThe single−line macros __NASM_MAJOR__, __NASM_MINOR__, __NASM_SUBMINOR__ and___NASM_PATCHLEVEL__ expand to the major, minor, subminor and patch level parts of theversion number of NASM being used. So, under NASM 0.98.32p1 for example,__NASM_MAJOR__ would be defined to be 0, __NASM_MINOR__ would be defined as 98,__NASM_SUBMINOR__ would be defined to 32, and ___NASM_PATCHLEVEL__ would bedefined as 1.

4.8.2 __NASM_VERSION_ID__: NASM Version IDThe single−line macro __NASM_VERSION_ID__ expands to a dword integer representing the fullversion number of the version of nasm being used. The value is the equivalent to__NASM_MAJOR__, __NASM_MINOR__, __NASM_SUBMINOR__ and___NASM_PATCHLEVEL__ concatenated to produce a single doubleword. Hence, for 0.98.32p1,the returned number would be equivalent to:

dd 0x00622001

or

db 1,32,98,0

Note that the above lines are generate exactly the same code, the second line is used just to give anindication of the order that the separate values will be present in memory.

4.8.3 __NASM_VER__: NASM Version stringThe single−line macro __NASM_VER__ expands to a string which defines the version number ofnasm being used. So, under NASM 0.98.32 for example,

db __NASM_VER__

would expand to

db "0.98.32"

4.8.4 __FILE__ and __LINE__ : File Name and Line NumberLike the C preprocessor, NASM allows the user to find out the file name and line numbercontaining the current instruction. The macro __FILE__ expands to a string constant giving thename of the current input file (which may change through the course of assembly if %includedirectives are used), and __LINE__ expands to a numeric constant giving the current line numberin the input file.

These macros could be used, for example, to communicate debugging information to a macro, sinceinvoking __LINE__ inside a macro definition (either single−line or multi−line) will return the linenumber of the macro call, rather than definition. So to determine where in a piece of code a crash isoccurring, for example, one could write a routine stillhere , which is passed a line number inEAX and outputs something like ‘line 155: still here’. You could then write a macro

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%macro notdeadyet 0

push eax mov eax,__LINE__ call stillhere pop eax

%endmacro

and then pepper your code with calls to notdeadyet until you find the crash point.

4.8.5 STRUC and ENDSTRUC: Declaring Structure Data TypesThe core of NASM contains no intrinsic means of defining data structures; instead, the preprocessoris sufficiently powerful that data structures can be implemented as a set of macros. The macrosSTRUC and ENDSTRUC are used to define a structure data type.

STRUC takes one parameter, which is the name of the data type. This name is defined as a symbolwith the value zero, and also has the suffix _size appended to it and is then defined as an EQUgiving the size of the structure. Once STRUC has been issued, you are defining the structure, andshould define fields using the RESB family of pseudo−instructions, and then invoke ENDSTRUC tofinish the definition.

For example, to define a structure called mytype containing a longword, a word, a byte and astring of bytes, you might code

struc mytype

mt_long: resd 1 mt_word: resw 1 mt_byte: resb 1 mt_str: resb 32

endstruc

The above code defines six symbols: mt_long as 0 (the offset from the beginning of a mytypestructure to the longword field), mt_word as 4, mt_byte as 6, mt_str as 7, mytype_size as39, and mytype itself as zero.

The reason why the structure type name is defined at zero is a side effect of allowing structures towork with the local label mechanism: if your structure members tend to have the same names inmore than one structure, you can define the above structure like this:

struc mytype

.long: resd 1 .word: resw 1 .byte: resb 1 .str: resb 32

endstruc

This defines the offsets to the structure fields as mytype.long , mytype.word , mytype.byteand mytype.str .

NASM, since it has no intrinsic structure support, does not support any form of period notation torefer to the elements of a structure once you have one (except the above local−label notation), socode such as mov ax,[mystruc.mt_word] is not valid. mt_word is a constant just like any

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other constant, so the correct syntax is mov ax,[mystruc+mt_word] ormov ax,[mystruc+mytype.word] .

4.8.6 ISTRUC, AT and IEND: Declaring Instances of StructuresHaving defined a structure type, the next thing you typically want to do is to declare instances ofthat structure in your data segment. NASM provides an easy way to do this in the ISTRUCmechanism. To declare a structure of type mytype in a program, you code something like this:

mystruc: istruc mytype

at mt_long, dd 123456 at mt_word, dw 1024 at mt_byte, db ’x’ at mt_str, db ’hello, world’, 13, 10, 0

iend

The function of the AT macro is to make use of the TIMES prefix to advance the assembly positionto the correct point for the specified structure field, and then to declare the specified data. Thereforethe structure fields must be declared in the same order as they were specified in the structuredefinition.

If the data to go in a structure field requires more than one source line to specify, the remainingsource lines can easily come after the AT line. For example:

at mt_str, db 123,134,145,156,167,178,189 db 190,100,0

Depending on personal taste, you can also omit the code part of the AT line completely, and start thestructure field on the next line:

at mt_str db ’hello, world’ db 13,10,0

4.8.7 ALIGN and ALIGNB: Data AlignmentThe ALIGN and ALIGNB macros provides a convenient way to align code or data on a word,longword, paragraph or other boundary. (Some assemblers call this directive EVEN.) The syntax ofthe ALIGN and ALIGNB macros is

align 4 ; align on 4−byte boundary align 16 ; align on 16−byte boundary align 8,db 0 ; pad with 0s rather than NOPs align 4,resb 1 ; align to 4 in the BSS alignb 4 ; equivalent to previous line

Both macros require their first argument to be a power of two; they both compute the number ofadditional bytes required to bring the length of the current section up to a multiple of that power oftwo, and then apply the TIMES prefix to their second argument to perform the alignment.

If the second argument is not specified, the default for ALIGN is NOP, and the default for ALIGNBis RESB 1. So if the second argument is specified, the two macros are equivalent. Normally, youcan just use ALIGN in code and data sections and ALIGNB in BSS sections, and never need thesecond argument except for special purposes.

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ALIGN and ALIGNB, being simple macros, perform no error checking: they cannot warn you iftheir first argument fails to be a power of two, or if their second argument generates more than onebyte of code. In each of these cases they will silently do the wrong thing.

ALIGNB (or ALIGN with a second argument of RESB 1) can be used within structure definitions:

struc mytype2

mt_byte: resb 1 alignb 2 mt_word: resw 1 alignb 4 mt_long: resd 1 mt_str: resb 32

endstruc

This will ensure that the structure members are sensibly aligned relative to the base of the structure.

A final caveat: ALIGN and ALIGNB work relative to the beginning of the section, not the beginningof the address space in the final executable. Aligning to a 16−byte boundary when the sectionyou’re in is only guaranteed to be aligned to a 4−byte boundary, for example, is a waste of effort.Again, NASM does not check that the section’s alignment characteristics are sensible for the use ofALIGN or ALIGNB.

4.9 TASM Compatible Preprocessor DirectivesThe following preprocessor directives may only be used when TASM compatibility is turned onusing the −t command line switch (This switch is described in section 2.1.16.)

• %arg (see section 4.9.1)

• %stacksize (see section 4.9.2)

• %local (see section 4.9.3)

4.9.1 %arg DirectiveThe %arg directive is used to simplify the handling of parameters passed on the stack. Stack basedparameter passing is used by many high level languages, including C, C++ and Pascal.

While NASM comes with macros which attempt to duplicate this functionality (see section 7.4.5),the syntax is not particularly convenient to use and is not TASM compatible. Here is an examplewhich shows the use of %arg without any external macros:

some_function:

%push mycontext ; save the current context %stacksize large ; tell NASM to use bp %arg i:word, j_ptr:word

mov ax,[i] mov bx,[j_ptr] add ax,[bx] ret

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%pop ; restore original context

This is similar to the procedure defined in section 7.4.5 and adds the value in i to the value pointedto by j_ptr and returns the sum in the ax register. See section 4.7.1 for an explanation of push andpop and the use of context stacks.

4.9.2 %stacksize DirectiveThe %stacksize directive is used in conjunction with the %arg (see section 4.9.1) and the%local (see section 4.9.3) directives. It tells NASM the default size to use for subsequent %argand %local directives. The %stacksize directive takes one required argument which is one offlat , large or small .

%stacksize flat

This form causes NASM to use stack−based parameter addressing relative to ebp and it assumesthat a near form of call was used to get to this label (i.e. that eip is on the stack).

%stacksize large

This form uses bp to do stack−based parameter addressing and assumes that a far form of call wasused to get to this address (i.e. that ip and cs are on the stack).

%stacksize small

This form also uses bp to address stack parameters, but it is different from large because it alsoassumes that the old value of bp is pushed onto the stack (i.e. it expects an ENTER instruction). Inother words, it expects that bp , ip and cs are on the top of the stack, underneath any local spacewhich may have been allocated by ENTER. This form is probably most useful when used incombination with the %local directive (see section 4.9.3).

4.9.3 %local DirectiveThe %local directive is used to simplify the use of local temporary stack variables allocated in astack frame. Automatic local variables in C are an example of this kind of variable. The %localdirective is most useful when used with the %stacksize (see section 4.9.2 and is also compatiblewith the %arg directive (see section 4.9.1). It allows simplified reference to variables on the stackwhich have been allocated typically by using the ENTER instruction (see section B.4.65 for adescription of that instruction). An example of its use is the following:

silly_swap:

%push mycontext ; save the current context %stacksize small ; tell NASM to use bp %assign %$localsize 0 ; see text for explanation %local old_ax:word, old_dx:word

enter %$localsize,0 ; see text for explanation mov [old_ax],ax ; swap ax & bx mov [old_dx],dx ; and swap dx & cx mov ax,bx mov dx,cx mov bx,[old_ax] mov cx,[old_dx] leave ; restore old bp ret ;

%pop ; restore original context

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The %$localsize variable is used internally by the %local directive and must be definedwithin the current context before the %local directive may be used. Failure to do so will result inone expression syntax error for each %local variable declared. It then may be used in theconstruction of an appropriately sized ENTER instruction as shown in the example.

4.10 Other Preprocessor DirectivesNASM also has preprocessor directives which allow access to information from external sources.Currently they include:

The following preprocessor directive is supported to allow NASM to correctly handle output of thecpp C language preprocessor.

• %line enables NAsM to correctly handle the output of the cpp C language preprocessor (seesection 4.10.1).

• %! enables NASM to read in the value of an environment variable, which can then be used inyour program (see section 4.10.2).

4.10.1 %line DirectiveThe %line directive is used to notify NASM that the input line corresponds to a specific linenumber in another file. Typically this other file would be an original source file, with the currentNASM input being the output of a pre−processor. The %line directive allows NASM to outputmessages which indicate the line number of the original source file, instead of the file that is beingread by NASM.

This preprocessor directive is not generally of use to programmers, by may be of interest topreprocessor authors. The usage of the %line preprocessor directive is as follows:

%line nnn[+mmm] [filename]

In this directive, nnn indentifies the line of the original source file which this line corresponds to.mmm is an optional parameter which specifies a line increment value; each line of the input file readin is considered to correspond to mmm lines of the original source file. Finally, filename is anoptional parameter which specifies the file name of the original source file.

After reading a %line preprocessor directive, NASM will report all file name and line numbersrelative to the values specified therein.

4.10.2 %!<env> : Read an environment variable.The %!<env> directive makes it possible to read the value of an environment variable at assemblytime. This could, for example, be used to store the contents of an environment variable into a string,which could be used at some other point in your code.

For example, suppose that you have an environment variable FOO, and you want the contents ofFOO to be embedded in your program. You could do that as follows:

%define FOO %!FOO %define quote ’

tmpstr db quote FOO quote

At the time of writing, this will generate an "unterminated string" warning at the time of defining"quote", and it will add a space before and after the string that is read in. I was unable to find asimple workaround (although a workaround can be created using a multi−line macro), so I believethat you will need to either learn how to create more complex macros, or allow for the extra spacesif you make use of this feature in that way.

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Chapter 5: Assembler Directives

NASM, though it attempts to avoid the bureaucracy of assemblers like MASM and TASM, isnevertheless forced to support a few directives. These are described in this chapter.

NASM’s directives come in two types: user−level directives and primitive directives. Typically,each directive has a user−level form and a primitive form. In almost all cases, we recommend thatusers use the user−level forms of the directives, which are implemented as macros which call theprimitive forms.

Primitive directives are enclosed in square brackets; user−level directives are not.

In addition to the universal directives described in this chapter, each object file format canoptionally supply extra directives in order to control particular features of that file format. Theseformat−specific directives are documented along with the formats that implement them, in chapter 6.

5.1 BITS : Specifying Target Processor ModeThe BITS directive specifies whether NASM should generate code designed to run on a processoroperating in 16−bit mode, or code designed to run on a processor operating in 32−bit mode. Thesyntax is BITS 16 or BITS 32 .

In most cases, you should not need to use BITS explicitly. The aout , coff , elf and win32object formats, which are designed for use in 32−bit operating systems, all cause NASM to select32−bit mode by default. The obj object format allows you to specify each segment you define aseither USE16 or USE32, and NASM will set its operating mode accordingly, so the use of theBITS directive is once again unnecessary.

The most likely reason for using the BITS directive is to write 32−bit code in a flat binary file; thisis because the bin output format defaults to 16−bit mode in anticipation of it being used mostfrequently to write DOS .COM programs, DOS .SYS device drivers and boot loader software.

You do not need to specify BITS 32 merely in order to use 32−bit instructions in a 16−bit DOSprogram; if you do, the assembler will generate incorrect code because it will be writing codetargeted at a 32−bit platform, to be run on a 16−bit one.

When NASM is in BITS 16 state, instructions which use 32−bit data are prefixed with an 0x66byte, and those referring to 32−bit addresses have an 0x67 prefix. In BITS 32 state, the reverse istrue: 32−bit instructions require no prefixes, whereas instructions using 16−bit data need an 0x66and those working on 16−bit addresses need an 0x67.

The BITS directive has an exactly equivalent primitive form, [BITS 16] and [BITS 32] . Theuser−level form is a macro which has no function other than to call the primitive form.

5.1.1 USE16 & USE32: Aliases for BITSThe ‘USE16’ and ‘USE32’ directives can be used in place of ‘BITS 16 ’ and ‘BITS 32 ’, forcompatibility with other assemblers.

5.2 SECTION or SEGMENT: Changing and Defining SectionsThe SECTION directive (SEGMENT is an exactly equivalent synonym) changes which section ofthe output file the code you write will be assembled into. In some object file formats, the numberand names of sections are fixed; in others, the user may make up as many as they wish. Hence

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SECTION may sometimes give an error message, or may define a new section, if you try to switchto a section that does not (yet) exist.

The Unix object formats, and the bin object format, all support the standardised section names.text , .data and .bss for the code, data and uninitialised−data sections. The obj format, bycontrast, does not recognise these section names as being special, and indeed will strip off theleading period of any section name that has one.

5.2.1 The __SECT__ MacroThe SECTION directive is unusual in that its user−level form functions differently from itsprimitive form. The primitive form, [SECTION xyz] , simply switches the current target sectionto the one given. The user−level form, SECTION xyz , however, first defines the single−linemacro __SECT__ to be the primitive [SECTION] directive which it is about to issue, and thenissues it. So the user−level directive

SECTION .text

expands to the two lines

%define __SECT__ [SECTION .text] [SECTION .text]

Users may find it useful to make use of this in their own macros. For example, the writefilemacro defined in section 4.3.3 can be usefully rewritten in the following more sophisticated form:

%macro writefile 2+

[section .data]

%%str: db %2 %%endstr:

__SECT__

mov dx,%%str mov cx,%%endstr−%%str mov bx,%1 mov ah,0x40 int 0x21

%endmacro

This form of the macro, once passed a string to output, first switches temporarily to the data sectionof the file, using the primitive form of the SECTION directive so as not to modify __SECT__. Itthen declares its string in the data section, and then invokes __SECT__ to switch back towhichever section the user was previously working in. It thus avoids the need, in the previousversion of the macro, to include a JMP instruction to jump over the data, and also does not fail if, ina complicated OBJ format module, the user could potentially be assembling the code in any ofseveral separate code sections.

5.3 ABSOLUTE: Defining Absolute LabelsThe ABSOLUTE directive can be thought of as an alternative form of SECTION: it causes thesubsequent code to be directed at no physical section, but at the hypothetical section starting at thegiven absolute address. The only instructions you can use in this mode are the RESB family.

ABSOLUTE is used as follows:

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absolute 0x1A

kbuf_chr resw 1 kbuf_free resw 1 kbuf resw 16

This example describes a section of the PC BIOS data area, at segment address 0x40: the abovecode defines kbuf_chr to be 0x1A, kbuf_free to be 0x1C, and kbuf to be 0x1E.

The user−level form of ABSOLUTE, like that of SECTION, redefines the __SECT__ macro whenit is invoked.

STRUC and ENDSTRUC are defined as macros which use ABSOLUTE (and also __SECT__).

ABSOLUTE doesn’t have to take an absolute constant as an argument: it can take an expression(actually, a critical expression: see section 3.8) and it can be a value in a segment. For example, aTSR can re−use its setup code as run−time BSS like this:

org 100h ; it’s a .COM program

jmp setup ; setup code comes last

; the resident part of the TSR goes here setup: ; now write the code that installs the TSR here

absolute setup

runtimevar1 resw 1 runtimevar2 resd 20

tsr_end:

This defines some variables ‘on top of’ the setup code, so that after the setup has finished running,the space it took up can be re−used as data storage for the running TSR. The symbol ‘tsr_end’ canbe used to calculate the total size of the part of the TSR that needs to be made resident.

5.4 EXTERN: Importing Symbols from Other ModulesEXTERN is similar to the MASM directive EXTRN and the C keyword extern : it is used todeclare a symbol which is not defined anywhere in the module being assembled, but is assumed tobe defined in some other module and needs to be referred to by this one. Not every object−fileformat can support external variables: the bin format cannot.

The EXTERN directive takes as many arguments as you like. Each argument is the name of asymbol:

extern _printf extern _sscanf,_fscanf

Some object−file formats provide extra features to the EXTERN directive. In all cases, the extrafeatures are used by suffixing a colon to the symbol name followed by object−format specific text.For example, the obj format allows you to declare that the default segment base of an externalshould be the group dgroup by means of the directive

extern _variable:wrt dgroup

The primitive form of EXTERN differs from the user−level form only in that it can take only oneargument at a time: the support for multiple arguments is implemented at the preprocessor level.

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You can declare the same variable as EXTERN more than once: NASM will quietly ignore thesecond and later redeclarations. You can’t declare a variable as EXTERN as well as something else,though.

5.5 GLOBAL: Exporting Symbols to Other ModulesGLOBAL is the other end of EXTERN: if one module declares a symbol as EXTERN and refers to it,then in order to prevent linker errors, some other module must actually define the symbol anddeclare it as GLOBAL. Some assemblers use the name PUBLIC for this purpose.

The GLOBAL directive applying to a symbol must appear before the definition of the symbol.

GLOBAL uses the same syntax as EXTERN, except that it must refer to symbols which are definedin the same module as the GLOBAL directive. For example:

global _main _main: ; some code

GLOBAL, like EXTERN, allows object formats to define private extensions by means of a colon. Theelf object format, for example, lets you specify whether global data items are functions or data:

global hashlookup:function, hashtable:data

Like EXTERN, the primitive form of GLOBAL differs from the user−level form only in that it cantake only one argument at a time.

5.6 COMMON: Defining Common Data AreasThe COMMON directive is used to declare common variables. A common variable is much like aglobal variable declared in the uninitialised data section, so that

common intvar 4

is similar in function to

global intvar section .bss

intvar resd 1

The difference is that if more than one module defines the same common variable, then at link timethose variables will be merged, and references to intvar in all modules will point at the samepiece of memory.

Like GLOBAL and EXTERN, COMMON supports object−format specific extensions. For example, theobj format allows common variables to be NEAR or FAR, and the elf format allows you tospecify the alignment requirements of a common variable:

common commvar 4:near ; works in OBJ common intarray 100:4 ; works in ELF: 4 byte aligned

Once again, like EXTERN and GLOBAL, the primitive form of COMMON differs from the user−levelform only in that it can take only one argument at a time.

5.7 CPU: Defining CPU DependenciesThe CPU directive restricts assembly to those instructions which are available on the specified CPU.

Options are:

• CPU 8086 Assemble only 8086 instruction set

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• CPU 186 Assemble instructions up to the 80186 instruction set



• CPU 486 486 instruction set

• CPU 586 Pentium instruction set

• CPU PENTIUM Same as 586

• CPU 686 P6 instruction set

• CPU PPRO Same as 686

• CPU P2 Same as 686

• CPU P3 Pentium III and Katmai instruction sets

• CPU KATMAI Same as P3

• CPU P4 Pentium 4 (Willamette) instruction set

• CPU WILLAMETTE Same as P4

• CPU IA64 IA64 CPU (in x86 mode) instruction set

All options are case insensitive. All instructions will be selected only if they apply to the selectedCPU or lower. By default, all instructions are available.

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Chapter 6: Output Formats

NASM is a portable assembler, designed to be able to compile on any ANSI C−supporting platformand produce output to run on a variety of Intel x86 operating systems. For this reason, it has a largenumber of available output formats, selected using the −f option on the NASM command line.Each of these formats, along with its extensions to the base NASM syntax, is detailed in this chapter.

As stated in section 2.1.1, NASM chooses a default name for your output file based on the input filename and the chosen output format. This will be generated by removing the extension (.asm , .s ,or whatever you like to use) from the input file name, and substituting an extension defined by theoutput format. The extensions are given with each format below.

6.1 bin : Flat−Form Binary OutputThe bin format does not produce object files: it generates nothing in the output file except the codeyou wrote. Such ‘pure binary’ files are used by MS−DOS: .COM executables and .SYS devicedrivers are pure binary files. Pure binary output is also useful for operating system and boot loaderdevelopment.

The bin format supports multiple section names. For details of how nasm handles sections in thebin format, see section 6.1.3.

Using the bin format puts NASM by default into 16−bit mode (see section 5.1). In order to usebin to write 32−bit code such as an OS kernel, you need to explicitly issue the BITS 32 directive.

bin has no default output file name extension: instead, it leaves your file name as it is once theoriginal extension has been removed. Thus, the default is for NASM to assemble binprog.asminto a binary file called binprog .

6.1.1 ORG: Binary File Program OriginThe bin format provides an additional directive to the list given in chapter 5: ORG. The function ofthe ORG directive is to specify the origin address which NASM will assume the program begins atwhen it is loaded into memory.

For example, the following code will generate the longword 0x00000104 :

org 0x100 dd label label:

Unlike the ORG directive provided by MASM−compatible assemblers, which allows you to jumparound in the object file and overwrite code you have already generated, NASM’s ORG does exactlywhat the directive says: origin. Its sole function is to specify one offset which is added to all internaladdress references within the section; it does not permit any of the trickery that MASM’s versiondoes. See section 10.1.3 for further comments.

6.1.2 bin Extensions to the SECTION DirectiveThe bin output format extends the SECTION (or SEGMENT) directive to allow you to specify thealignment requirements of segments. This is done by appending the ALIGN qualifier to the end ofthe section−definition line. For example,

section .data align=16

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switches to the section .data and also specifies that it must be aligned on a 16−byte boundary.

The parameter to ALIGN specifies how many low bits of the section start address must be forced tozero. The alignment value given may be any power of two.

6.1.3 Multisection support for the BIN format.The bin format allows the use of multiple sections, which are ordered according to a few basicrules.

• Any code which comes before an explicit SECTION directive is directed by default into the.text section.

• If a .text section is not given an ORG statement, it is allocated ORG 0 by default.

• Sections which have an ORG statement, explicit or implicit, are placed in the order of the ORGstatement. The code is padded with 0s to give the correct offsets within the output file.

• If a section has multiple ORG statements, the last ORG statement is applied to the entire section,without affecting the order in which the separate parts of the section are put together at assemblytime.

• Sections without an ORG statement will be placed after those which do have one, and they will beplaced in the order that they are first declared.

• The .data section does not follow any special rules, unlike the .text and .bss sections.

• The .bss section will be placed after all other sections.

• All sections are aligned on dword boundaries, unless a higher level of alignment has beenspecified.

• Sections cannot overlap.

6.2 obj : Microsoft OMF Object FilesThe obj file format (NASM calls it obj rather than omf for historical reasons) is the one producedby MASM and TASM, which is typically fed to 16−bit DOS linkers to produce .EXE files. It isalso the format used by OS/2.

obj provides a default output file−name extension of .obj .

obj is not exclusively a 16−bit format, though: NASM has full support for the 32−bit extensions tothe format. In particular, 32−bit obj format files are used by Borland’s Win32 compilers, instead ofusing Microsoft’s newer win32 object file format.

The obj format does not define any special segment names: you can call your segments anythingyou like. Typical names for segments in obj format files are CODE, DATA and BSS.

If your source file contains code before specifying an explicit SEGMENT directive, then NASM willinvent its own segment called __NASMDEFSEG for you.

When you define a segment in an obj file, NASM defines the segment name as a symbol as well,so that you can access the segment address of the segment. So, for example:

segment data

dvar: dw 1234

segment code

function:

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mov ax,data ; get segment address of data mov ds,ax ; and move it into DS inc word [dvar] ; now this reference will work ret

The obj format also enables the use of the SEG and WRT operators, so that you can write codewhich does things like

extern foo

mov ax,seg foo ; get preferred segment of foo mov ds,ax mov ax,data ; a different segment mov es,ax mov ax,[ds:foo] ; this accesses ‘foo’ mov [es:foo wrt data],bx ; so does this

6.2.1 obj Extensions to the SEGMENT DirectiveThe obj output format extends the SEGMENT (or SECTION) directive to allow you to specifyvarious properties of the segment you are defining. This is done by appending extra qualifiers to theend of the segment−definition line. For example,

segment code private align=16

defines the segment code , but also declares it to be a private segment, and requires that the portionof it described in this code module must be aligned on a 16−byte boundary.

The available qualifiers are:

• PRIVATE, PUBLIC, COMMON and STACK specify the combination characteristics of thesegment. PRIVATE segments do not get combined with any others by the linker; PUBLIC andSTACK segments get concatenated together at link time; and COMMON segments all get overlaidon top of each other rather than stuck end−to−end.

• ALIGN is used, as shown above, to specify how many low bits of the segment start address mustbe forced to zero. The alignment value given may be any power of two from 1 to 4096; in reality,the only values supported are 1, 2, 4, 16, 256 and 4096, so if 8 is specified it will be rounded upto 16, and 32, 64 and 128 will all be rounded up to 256, and so on. Note that alignment to4096−byte boundaries is a PharLap extension to the format and may not be supported by alllinkers.

• CLASS can be used to specify the segment class; this feature indicates to the linker that segmentsof the same class should be placed near each other in the output file. The class name can be anyword, e.g. CLASS=CODE.

• OVERLAY, like CLASS, is specified with an arbitrary word as an argument, and provides overlayinformation to an overlay−capable linker.

• Segments can be declared as USE16 or USE32, which has the effect of recording the choice inthe object file and also ensuring that NASM’s default assembly mode when assembling in thatsegment is 16−bit or 32−bit respectively.

• When writing OS/2 object files, you should declare 32−bit segments as FLAT, which causes thedefault segment base for anything in the segment to be the special group FLAT, and also definesthe group if it is not already defined.

• The obj file format also allows segments to be declared as having a pre−defined absolutesegment address, although no linkers are currently known to make sensible use of this feature;nevertheless, NASM allows you to declare a segment such as

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SEGMENT SCREEN ABSOLUTE=0xB800 if you need to. The ABSOLUTE and ALIGNkeywords are mutually exclusive.

NASM’s default segment attributes are PUBLIC, ALIGN=1, no class, no overlay, and USE16.

6.2.2 GROUP: Defining Groups of SegmentsThe obj format also allows segments to be grouped, so that a single segment register can be usedto refer to all the segments in a group. NASM therefore supplies the GROUP directive, whereby youcan code

segment data

; some data

segment bss

; some uninitialised data

group dgroup data bss

which will define a group called dgroup to contain the segments data and bss . Like SEGMENT,GROUP causes the group name to be defined as a symbol, so that you can refer to a variable var inthe data segment as var wrt data or as var wrt dgroup , depending on which segmentvalue is currently in your segment register.

If you just refer to var , however, and var is declared in a segment which is part of a group, thenNASM will default to giving you the offset of var from the beginning of the group, not thesegment. Therefore SEG var , also, will return the group base rather than the segment base.

NASM will allow a segment to be part of more than one group, but will generate a warning if youdo this. Variables declared in a segment which is part of more than one group will default to beingrelative to the first group that was defined to contain the segment.

A group does not have to contain any segments; you can still make WRT references to a group whichdoes not contain the variable you are referring to. OS/2, for example, defines the special groupFLAT with no segments in it.

6.2.3 UPPERCASE: Disabling Case Sensitivity in OutputAlthough NASM itself is case sensitive, some OMF linkers are not; therefore it can be useful forNASM to output single−case object files. The UPPERCASE format−specific directive causes allsegment, group and symbol names that are written to the object file to be forced to upper case justbefore being written. Within a source file, NASM is still case−sensitive; but the object file can bewritten entirely in upper case if desired.

UPPERCASE is used alone on a line; it requires no parameters.

6.2.4 IMPORT: Importing DLL SymbolsThe IMPORT format−specific directive defines a symbol to be imported from a DLL, for use if youare writing a DLL’s import library in NASM. You still need to declare the symbol as EXTERN aswell as using the IMPORT directive.

The IMPORT directive takes two required parameters, separated by white space, which are(respectively) the name of the symbol you wish to import and the name of the library you wish toimport it from. For example:

import WSAStartup wsock32.dll

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A third optional parameter gives the name by which the symbol is known in the library you areimporting it from, in case this is not the same as the name you wish the symbol to be known by toyour code once you have imported it. For example:

import asyncsel wsock32.dll WSAAsyncSelect

6.2.5 EXPORT: Exporting DLL SymbolsThe EXPORT format−specific directive defines a global symbol to be exported as a DLL symbol,for use if you are writing a DLL in NASM. You still need to declare the symbol as GLOBAL as wellas using the EXPORT directive.

EXPORT takes one required parameter, which is the name of the symbol you wish to export, as itwas defined in your source file. An optional second parameter (separated by white space from thefirst) gives the external name of the symbol: the name by which you wish the symbol to be knownto programs using the DLL. If this name is the same as the internal name, you may leave the secondparameter off.

Further parameters can be given to define attributes of the exported symbol. These parameters, likethe second, are separated by white space. If further parameters are given, the external name mustalso be specified, even if it is the same as the internal name. The available attributes are:

• resident indicates that the exported name is to be kept resident by the system loader. This isan optimisation for frequently used symbols imported by name.

• nodata indicates that the exported symbol is a function which does not make use of anyinitialised data.

• parm=NNN, where NNN is an integer, sets the number of parameter words for the case in whichthe symbol is a call gate between 32−bit and 16−bit segments.

• An attribute which is just a number indicates that the symbol should be exported with anidentifying number (ordinal), and gives the desired number.

For example:

export myfunc export myfunc TheRealMoreFormalLookingFunctionName export myfunc myfunc 1234 ; export by ordinal export myfunc myfunc resident parm=23 nodata

6.2.6 ..start : Defining the Program Entry PointOMF linkers require exactly one of the object files being linked to define the program entry point,where execution will begin when the program is run. If the object file that defines the entry point isassembled using NASM, you specify the entry point by declaring the special symbol ..start atthe point where you wish execution to begin.

6.2.7 obj Extensions to the EXTERN DirectiveIf you declare an external symbol with the directive

extern foo

then references such as mov ax,foo will give you the offset of foo from its preferred segmentbase (as specified in whichever module foo is actually defined in). So to access the contents offoo you will usually need to do something like

mov ax,seg foo ; get preferred segment base mov es,ax ; move it into ES mov ax,[es:foo] ; and use offset ‘foo’ from it

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This is a little unwieldy, particularly if you know that an external is going to be accessible from agiven segment or group, say dgroup . So if DS already contained dgroup , you could simply code

mov ax,[foo wrt dgroup]

However, having to type this every time you want to access foo can be a pain; so NASM allowsyou to declare foo in the alternative form

extern foo:wrt dgroup

This form causes NASM to pretend that the preferred segment base of foo is in fact dgroup ; sothe expression seg foo will now return dgroup , and the expression foo is equivalent tofoo wrt dgroup .

This default−WRT mechanism can be used to make externals appear to be relative to any group orsegment in your program. It can also be applied to common variables: see section 6.2.8.

6.2.8 obj Extensions to the COMMON DirectiveThe obj format allows common variables to be either near or far; NASM allows you to specifywhich your variables should be by the use of the syntax

common nearvar 2:near ; ‘nearvar’ is a near common common farvar 10:far ; and ‘farvar’ is far

Far common variables may be greater in size than 64Kb, and so the OMF specification says thatthey are declared as a number of elements of a given size. So a 10−byte far common variable couldbe declared as ten one−byte elements, five two−byte elements, two five−byte elements or oneten−byte element.

Some OMF linkers require the element size, as well as the variable size, to match when resolvingcommon variables declared in more than one module. Therefore NASM must allow you to specifythe element size on your far common variables. This is done by the following syntax:

common c_5by2 10:far 5 ; two five−byte elements common c_2by5 10:far 2 ; five two−byte elements

If no element size is specified, the default is 1. Also, the FAR keyword is not required when anelement size is specified, since only far commons may have element sizes at all. So the abovedeclarations could equivalently be

common c_5by2 10:5 ; two five−byte elements common c_2by5 10:2 ; five two−byte elements

In addition to these extensions, the COMMON directive in obj also supports default−WRTspecification like EXTERN does (explained in section 6.2.7). So you can also declare things like

common foo 10:wrt dgroup common bar 16:far 2:wrt data common baz 24:wrt data:6

6.3 win32 : Microsoft Win32 Object FilesThe win32 output format generates Microsoft Win32 object files, suitable for passing to Microsoftlinkers such as Visual C++. Note that Borland Win32 compilers do not use this format, but use objinstead (see section 6.2).

win32 provides a default output file−name extension of .obj .

Note that although Microsoft say that Win32 object files follow the COFF (Common Object FileFormat) standard, the object files produced by Microsoft Win32 compilers are not compatible withCOFF linkers such as DJGPP’s, and vice versa. This is due to a difference of opinion over the

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precise semantics of PC−relative relocations. To produce COFF files suitable for DJGPP, useNASM’s coff output format; conversely, the coff format does not produce object files thatWin32 linkers can generate correct output from.

6.3.1 win32 Extensions to the SECTION DirectiveLike the obj format, win32 allows you to specify additional information on the SECTIONdirective line, to control the type and properties of sections you declare. Section types andproperties are generated automatically by NASM for the standard section names .text , .dataand .bss , but may still be overridden by these qualifiers.


• code , or equivalently text , defines the section to be a code section. This marks the section asreadable and executable, but not writable, and also indicates to the linker that the type of thesection is code.

• data and bss define the section to be a data section, analogously to code . Data sections aremarked as readable and writable, but not executable. data declares an initialised data section,whereas bss declares an uninitialised data section.

• rdata declares an initialised data section that is readable but not writable. Microsoft compilersuse this section to place constants in it.

• info defines the section to be an informational section, which is not included in the executablefile by the linker, but may (for example) pass information to the linker. For example, declaring aninfo –type section called .drectve causes the linker to interpret the contents of the section ascommand−line options.

• align= , used with a trailing number as in obj , gives the alignment requirements of the section.The maximum you may specify is 64: the Win32 object file format contains no means to requesta greater section alignment than this. If alignment is not explicitly specified, the defaults are16−byte alignment for code sections, 8−byte alignment for rdata sections and 4−byte alignmentfor data (and BSS) sections. Informational sections get a default alignment of 1 byte (noalignment), though the value does not matter.

The defaults assumed by NASM if you do not specify the above qualifiers are:

section .text code align=16 section .data data align=4 section .rdata rdata align=8 section .bss bss align=4

Any other section name is treated by default like .text .

6.4 coff : Common Object File FormatThe coff output type produces COFF object files suitable for linking with the DJGPP linker.

coff provides a default output file−name extension of .o .

The coff format supports the same extensions to the SECTION directive as win32 does, exceptthat the align qualifier and the info section type are not supported.

6.5 elf : Executable and Linkable Format Object FilesThe elf output format generates ELF32 (Executable and Linkable Format) object files, as used byLinux as well as Unix System V, including Solaris x86, UnixWare and SCO Unix. elf provides adefault output file−name extension of .o .

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6.5.1 elf Extensions to the SECTION DirectiveLike the obj format, elf allows you to specify additional information on the SECTION directiveline, to control the type and properties of sections you declare. Section types and properties aregenerated automatically by NASM for the standard section names .text , .data and .bss , butmay still be overridden by these qualifiers.


• alloc defines the section to be one which is loaded into memory when the program is run.noalloc defines it to be one which is not, such as an informational or comment section.

• exec defines the section to be one which should have execute permission when the program isrun. noexec defines it as one which should not.

• write defines the section to be one which should be writable when the program is run.nowrite defines it as one which should not.

• progbits defines the section to be one with explicit contents stored in the object file: anordinary code or data section, for example, nobits defines the section to be one with noexplicit contents given, such as a BSS section.

• align= , used with a trailing number as in obj , gives the alignment requirements of the section.

The defaults assumed by NASM if you do not specify the above qualifiers are:

section .text progbits alloc exec nowrite align=16 section .rodata progbits alloc noexec nowrite align=4 section .data progbits alloc noexec write align=4 section .bss nobits alloc noexec write align=4 section other progbits alloc noexec nowrite align=1

(Any section name other than .text , .rodata , .data and .bss is treated by default likeother in the above code.)

6.5.2 Position−Independent Code: elf Special Symbols and WRT

The ELF specification contains enough features to allow position−independent code (PIC) to bewritten, which makes ELF shared libraries very flexible. However, it also means NASM has to beable to generate a variety of strange relocation types in ELF object files, if it is to be an assemblerwhich can write PIC.

Since ELF does not support segment−base references, the WRT operator is not used for its normalpurpose; therefore NASM’s elf output format makes use of WRT for a different purpose, namelythe PIC−specific relocation types.

elf defines five special symbols which you can use as the right−hand side of the WRT operator toobtain PIC relocation types. They are ..gotpc , ..gotoff , ..got , ..plt and ..sym . Theirfunctions are summarised here:

• Referring to the symbol marking the global offset table base using wrt ..gotpc will end upgiving the distance from the beginning of the current section to the global offset table.(_GLOBAL_OFFSET_TABLE_ is the standard symbol name used to refer to the GOT.) So youwould then need to add $$ to the result to get the real address of the GOT.

• Referring to a location in one of your own sections using wrt ..gotoff will give the distancefrom the beginning of the GOT to the specified location, so that adding on the address of theGOT would give the real address of the location you wanted.

• Referring to an external or global symbol using wrt ..got causes the linker to build an entryin the GOT containing the address of the symbol, and the reference gives the distance from the

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beginning of the GOT to the entry; so you can add on the address of the GOT, load from theresulting address, and end up with the address of the symbol.

• Referring to a procedure name using wrt ..plt causes the linker to build a procedure linkagetable entry for the symbol, and the reference gives the address of the PLT entry. You can onlyuse this in contexts which would generate a PC−relative relocation normally (i.e. as thedestination for CALL or JMP), since ELF contains no relocation type to refer to PLT entriesabsolutely.

• Referring to a symbol name using wrt ..sym causes NASM to write an ordinary relocation,but instead of making the relocation relative to the start of the section and then adding on theoffset to the symbol, it will write a relocation record aimed directly at the symbol in question.The distinction is a necessary one due to a peculiarity of the dynamic linker.

A fuller explanation of how to use these relocation types to write shared libraries entirely in NASMis given in section 8.2.

6.5.3 elf Extensions to the GLOBAL DirectiveELF object files can contain more information about a global symbol than just its address: they cancontain the size of the symbol and its type as well. These are not merely debugger conveniences, butare actually necessary when the program being written is a shared library. NASM therefore supportssome extensions to the GLOBAL directive, allowing you to specify these features.

You can specify whether a global variable is a function or a data object by suffixing the name with acolon and the word function or data . (object is a synonym for data .) For example:

global hashlookup:function, hashtable:data

exports the global symbol hashlookup as a function and hashtable as a data object.

You can also specify the size of the data associated with the symbol, as a numeric expression(which may involve labels, and even forward references) after the type specifier. Like this:

global hashtable:data (hashtable.end − hashtable)

hashtable: db this,that,theother ; some data here .end:

This makes NASM automatically calculate the length of the table and place that information intothe ELF symbol table.

Declaring the type and size of global symbols is necessary when writing shared library code. Formore information, see section 8.2.4.

6.5.4 elf Extensions to the COMMON Directive ELF also allows you to specify alignment requirements on common variables. This is done byputting a number (which must be a power of two) after the name and size of the common variable,separated (as usual) by a colon. For example, an array of doublewords would benefit from 4−bytealignment:

common dwordarray 128:4

This declares the total size of the array to be 128 bytes, and requires that it be aligned on a 4−byteboundary.

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6.5.5 16−bit code and ELF The ELF32 specification doesn’t provide relocations for 8− and 16−bit values, but the GNU ldlinker adds these as an extension. NASM can generate GNU−compatible relocations, to allow16−bit code to be linked as ELF using GNU ld . If NASM is used with the−w+gnu−elf−extensions option, a warning is issued when one of these relocations isgenerated.

6.6 aout : Linux a.out Object FilesThe aout format generates a.out object files, in the form used by early Linux systems (currentLinux systems use ELF, see section 6.5.) These differ from other a.out object files in that themagic number in the first four bytes of the file is different; also, some implementations of a.out ,for example NetBSD’s, support position−independent code, which Linux’s implementation does not.

a.out provides a default output file−name extension of .o .

a.out is a very simple object format. It supports no special directives, no special symbols, no useof SEG or WRT, and no extensions to any standard directives. It supports only the three standardsection names .text , .data and .bss .

6.7 aoutb : NetBSD/FreeBSD/OpenBSD a.out Object FilesThe aoutb format generates a.out object files, in the form used by the various free BSD Unixclones, NetBSD, FreeBSD and OpenBSD. For simple object files, this object format is exactly thesame as aout except for the magic number in the first four bytes of the file. However, the aoutbformat supports position−independent code in the same way as the elf format, so you can use it towrite BSD shared libraries.

aoutb provides a default output file−name extension of .o .

aoutb supports no special directives, no special symbols, and only the three standard sectionnames .text , .data and .bss . However, it also supports the same use of WRT as elf does, toprovide position−independent code relocation types. See section 6.5.2 for full documentation of thisfeature.

aoutb also supports the same extensions to the GLOBAL directive as elf does: see section 6.5.3for documentation of this.

6.8 as86 : Minix/Linux as86 Object FilesThe Minix/Linux 16−bit assembler as86 has its own non−standard object file format. Although itscompanion linker ld86 produces something close to ordinary a.out binaries as output, the objectfile format used to communicate between as86 and ld86 is not itself a.out .

NASM supports this format, just in case it is useful, as as86 . as86 provides a default outputfile−name extension of .o .

as86 is a very simple object format (from the NASM user’s point of view). It supports no specialdirectives, no special symbols, no use of SEG or WRT, and no extensions to any standard directives.It supports only the three standard section names .text , .data and .bss .

6.9 rdf : Relocatable Dynamic Object File FormatThe rdf output format produces RDOFF object files. RDOFF (Relocatable Dynamic Object FileFormat) is a home−grown object−file format, designed alongside NASM itself and reflecting in itsfile format the internal structure of the assembler.

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RDOFF is not used by any well−known operating systems. Those writing their own systems,however, may well wish to use RDOFF as their object format, on the grounds that it is designedprimarily for simplicity and contains very little file−header bureaucracy.

The Unix NASM archive, and the DOS archive which includes sources, both contain an rdoffsubdirectory holding a set of RDOFF utilities: an RDF linker, an RDF static−library manager, anRDF file dump utility, and a program which will load and execute an RDF executable under Linux.

rdf supports only the standard section names .text , .data and .bss .

6.9.1 Requiring a Library: The LIBRARY DirectiveRDOFF contains a mechanism for an object file to demand a given library to be linked to themodule, either at load time or run time. This is done by the LIBRARY directive, which takes oneargument which is the name of the module:

library mylib.rdl

6.9.2 Specifying a Module Name: The MODULE DirectiveSpecial RDOFF header record is used to store the name of the module. It can be used, for example,by run−time loader to perform dynamic linking. MODULE directive takes one argument which is thename of current module:

module mymodname

Note that when you statically link modules and tell linker to strip the symbols from output file, allmodule names will be stripped too. To avoid it, you should start module names with $, like:

module $kernel.core

6.9.3 rdf Extensions to the GLOBAL directiveRDOFF global symbols can contain additional information needed by the static linker. You canmark a global symbol as exported, thus telling the linker do not strip it from target executable orlibrary file. Like in ELF, you can also specify whether an exported symbol is a procedure (function)or data object.

Suffixing the name with a colon and the word export you make the symbol exported:

global sys_open:export

To specify that exported symbol is a procedure (function), you add the word proc or functionafter declaration:

global sys_open:export proc

Similarly, to specify exported data object, add the word data or object to the directive:

global kernel_ticks:export data

6.10 dbg : Debugging FormatThe dbg output format is not built into NASM in the default configuration. If you are building yourown NASM executable from the sources, you can define OF_DBG in outform.h or on thecompiler command line, and obtain the dbg output format.

The dbg format does not output an object file as such; instead, it outputs a text file which contains acomplete list of all the transactions between the main body of NASM and the output−format backend module. It is primarily intended to aid people who want to write their own output drivers, sothat they can get a clearer idea of the various requests the main program makes of the output driver,and in what order they happen.

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For simple files, one can easily use the dbg format like this:

nasm −f dbg filename.asm

which will generate a diagnostic file called filename.dbg . However, this will not work well onfiles which were designed for a different object format, because each object format defines its ownmacros (usually user−level forms of directives), and those macros will not be defined in the dbgformat. Therefore it can be useful to run NASM twice, in order to do the preprocessing with thenative object format selected:

nasm −e −f rdf −o rdfprog.i rdfprog.asm nasm −a −f dbg rdfprog.i

This preprocesses rdfprog.asm into rdfprog.i , keeping the rdf object format selected inorder to make sure RDF special directives are converted into primitive form correctly. Then thepreprocessed source is fed through the dbg format to generate the final diagnostic output.

This workaround will still typically not work for programs intended for obj format, because theobj SEGMENT and GROUP directives have side effects of defining the segment and group names assymbols; dbg will not do this, so the program will not assemble. You will have to work around thatby defining the symbols yourself (using EXTERN, for example) if you really need to get a dbg traceof an obj –specific source file.

dbg accepts any section name and any directives at all, and logs them all to its output file.

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Chapter 7: Writing 16−bit Code (DOS, Windows 3/3.1)

This chapter attempts to cover some of the common issues encountered when writing 16−bit code torun under MS−DOS or Windows 3.x . It covers how to link programs to produce .EXE or .COMfiles, how to write .SYS device drivers, and how to interface assembly language code with 16−bitC compilers and with Borland Pascal.

7.1 Producing .EXE FilesAny large program written under DOS needs to be built as a .EXE file: only .EXE files have thenecessary internal structure required to span more than one 64K segment. Windows programs, also,have to be built as .EXE files, since Windows does not support the .COM format.

In general, you generate .EXE files by using the obj output format to produce one or more .OBJfiles, and then linking them together using a linker. However, NASM also supports the directgeneration of simple DOS .EXE files using the bin output format (by using DB and DW toconstruct the .EXE file header), and a macro package is supplied to do this. Thanks to Yann Guidonfor contributing the code for this.

NASM may also support .EXE natively as another output format in future releases.

7.1.1 Using the obj Format To Generate .EXE FilesThis section describes the usual method of generating .EXE files by linking .OBJ files together.

Most 16−bit programming language packages come with a suitable linker; if you have none ofthese, there is a free linker called VAL , available in LZH archive format from x2ftp.oulu.fi .An LZH archiver can be found at ftp.simtel.net . There is another ‘free’ linker (though thisone doesn’t come with sources) called FREELINK, available from www.pcorner.com . A third,djlink , written by DJ Delorie, is available at www.delorie.com . A fourth linker, ALINK ,written by Anthony A.J. Williams, is available at alink.sourceforge.net .

When linking several .OBJ files into a .EXE file, you should ensure that exactly one of them has astart point defined (using the ..start special symbol defined by the obj format: see section6.2.6). If no module defines a start point, the linker will not know what value to give theentry−point field in the output file header; if more than one defines a start point, the linker will notknow which value to use.

An example of a NASM source file which can be assembled to a .OBJ file and linked on its own toa .EXE is given here. It demonstrates the basic principles of defining a stack, initialising thesegment registers, and declaring a start point. This file is also provided in the test subdirectory ofthe NASM archives, under the name objexe.asm .

segment code

..start: mov ax,data mov ds,ax mov ax,stack mov ss,ax mov sp,stacktop

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http://alink.sourceforge.net

http://www.delorie.com/djgpp/16bit/djlink/

http://www.pcorner.com/tpc/old/3-101.html

ftp://ftp.simtel.net/pub/simtelnet/msdos/arcers

ftp://x2ftp.oulu.fi/pub/msdos/programming/lang/

This initial piece of code sets up DS to point to the data segment, and initialises SS and SP to pointto the top of the provided stack. Notice that interrupts are implicitly disabled for one instructionafter a move into SS, precisely for this situation, so that there’s no chance of an interrupt occurringbetween the loads of SS and SP and not having a stack to execute on.

Note also that the special symbol ..start is defined at the beginning of this code, which meansthat will be the entry point into the resulting executable file.

mov dx,hello mov ah,9 int 0x21

The above is the main program: load DS:DX with a pointer to the greeting message (hello isimplicitly relative to the segment data , which was loaded into DS in the setup code, so the fullpointer is valid), and call the DOS print−string function.

mov ax,0x4c00 int 0x21

This terminates the program using another DOS system call.

segment data

hello: db ’hello, world’, 13, 10, ’$’

The data segment contains the string we want to display.

segment stack stack resb 64 stacktop:

The above code declares a stack segment containing 64 bytes of uninitialised stack space, andpoints stacktop at the top of it. The directive segment stack stack defines a segmentcalled stack , and also of type STACK. The latter is not necessary to the correct running of theprogram, but linkers are likely to issue warnings or errors if your program has no segment of typeSTACK.

The above file, when assembled into a .OBJ file, will link on its own to a valid .EXE file, whichwhen run will print ‘hello, world’ and then exit.

7.1.2 Using the bin Format To Generate .EXE FilesThe .EXE file format is simple enough that it’s possible to build a .EXE file by writing apure−binary program and sticking a 32−byte header on the front. This header is simple enough thatit can be generated using DB and DW commands by NASM itself, so that you can use the bin outputformat to directly generate .EXE files.

Included in the NASM archives, in the misc subdirectory, is a file exebin.mac of macros. Itdefines three macros: EXE_begin , EXE_stack and EXE_end.

To produce a .EXE file using this method, you should start by using %include to load theexebin.mac macro package into your source file. You should then issue the EXE_begin macrocall (which takes no arguments) to generate the file header data. Then write code as normal for thebin format – you can use all three standard sections .text , .data and .bss . At the end of thefile you should call the EXE_end macro (again, no arguments), which defines some symbols tomark section sizes, and these symbols are referred to in the header code generated by EXE_begin .

In this model, the code you end up writing starts at 0x100 , just like a .COM file – in fact, if youstrip off the 32−byte header from the resulting .EXE file, you will have a valid .COM program. Allthe segment bases are the same, so you are limited to a 64K program, again just like a .COM file.

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Note that an ORG directive is issued by the EXE_begin macro, so you should not explicitly issueone of your own.

You can’t directly refer to your segment base value, unfortunately, since this would require arelocation in the header, and things would get a lot more complicated. So you should get yoursegment base by copying it out of CS instead.

On entry to your .EXE file, SS:SP are already set up to point to the top of a 2Kb stack. You canadjust the default stack size of 2Kb by calling the EXE_stack macro. For example, to change thestack size of your program to 64 bytes, you would call EXE_stack 64 .

A sample program which generates a .EXE file in this way is given in the test subdirectory of theNASM archive, as binexe.asm .

7.2 Producing .COM FilesWhile large DOS programs must be written as .EXE files, small ones are often better written as.COM files. .COM files are pure binary, and therefore most easily produced using the bin outputformat.

7.2.1 Using the bin Format To Generate .COM Files.COM files expect to be loaded at offset 100h into their segment (though the segment may change).Execution then begins at 100h , i.e. right at the start of the program. So to write a .COM program,you would create a source file looking like

org 100h

section .text

start: ; put your code here

section .data

; put data items here

section .bss

; put uninitialised data here

The bin format puts the .text section first in the file, so you can declare data or BSS itemsbefore beginning to write code if you want to and the code will still end up at the front of the filewhere it belongs.

The BSS (uninitialised data) section does not take up space in the .COM file itself: instead,addresses of BSS items are resolved to point at space beyond the end of the file, on the grounds thatthis will be free memory when the program is run. Therefore you should not rely on your BSS beinginitialised to all zeros when you run.

To assemble the above program, you should use a command line like

nasm myprog.asm −fbin −o myprog.com

The bin format would produce a file called myprog if no explicit output file name were specified,so you have to override it and give the desired file name.

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7.2.2 Using the obj Format To Generate .COM FilesIf you are writing a .COM program as more than one module, you may wish to assemble several.OBJ files and link them together into a .COM program. You can do this, provided you have alinker capable of outputting .COM files directly (TLINK does this), or alternatively a converterprogram such as EXE2BIN to transform the .EXE file output from the linker into a .COM file.

If you do this, you need to take care of several things:

• The first object file containing code should start its code segment with a line like RESB 100h .This is to ensure that the code begins at offset 100h relative to the beginning of the codesegment, so that the linker or converter program does not have to adjust address references withinthe file when generating the .COM file. Other assemblers use an ORG directive for this purpose,but ORG in NASM is a format−specific directive to the bin output format, and does not meanthe same thing as it does in MASM−compatible assemblers.

• You don’t need to define a stack segment.

• All your segments should be in the same group, so that every time your code or data references asymbol offset, all offsets are relative to the same segment base. This is because, when a .COMfile is loaded, all the segment registers contain the same value.

7.3 Producing .SYS FilesMS−DOS device drivers – .SYS files – are pure binary files, similar to .COM files, except that theystart at origin zero rather than 100h . Therefore, if you are writing a device driver using the binformat, you do not need the ORG directive, since the default origin for bin is zero. Similarly, if youare using obj , you do not need the RESB 100h at the start of your code segment.

.SYS files start with a header structure, containing pointers to the various routines inside the driverwhich do the work. This structure should be defined at the start of the code segment, even though itis not actually code.

For more information on the format of .SYS files, and the data which has to go in the headerstructure, a list of books is given in the Frequently Asked Questions list for the newsgroupcomp.os.msdos.programmer .

7.4 Interfacing to 16−bit C ProgramsThis section covers the basics of writing assembly routines that call, or are called from, C programs.To do this, you would typically write an assembly module as a .OBJ file, and link it with your Cmodules to produce a mixed−language program.

7.4.1 External Symbol NamesC compilers have the convention that the names of all global symbols (functions or data) theydefine are formed by prefixing an underscore to the name as it appears in the C program. So, forexample, the function a C programmer thinks of as printf appears to an assembly languageprogrammer as _printf . This means that in your assembly programs, you can define symbolswithout a leading underscore, and not have to worry about name clashes with C symbols.

If you find the underscores inconvenient, you can define macros to replace the GLOBAL andEXTERN directives as follows:

%macro cglobal 1

global _%1 %define %1 _%1

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%endmacro

%macro cextern 1

extern _%1 %define %1 _%1

%endmacro

(These forms of the macros only take one argument at a time; a %rep construct could solve this.)

If you then declare an external like this:

cextern printf

then the macro will expand it as

extern _printf %define printf _printf

Thereafter, you can reference printf as if it was a symbol, and the preprocessor will put theleading underscore on where necessary.

The cglobal macro works similarly. You must use cglobal before defining the symbol inquestion, but you would have had to do that anyway if you used GLOBAL.

7.4.2 Memory ModelsNASM contains no mechanism to support the various C memory models directly; you have to keeptrack yourself of which one you are writing for. This means you have to keep track of the followingthings:

• In models using a single code segment (tiny, small and compact), functions are near. This meansthat function pointers, when stored in data segments or pushed on the stack as functionarguments, are 16 bits long and contain only an offset field (the CS register never changes itsvalue, and always gives the segment part of the full function address), and that functions arecalled using ordinary near CALL instructions and return using RETN (which, in NASM, issynonymous with RET anyway). This means both that you should write your own routines toreturn with RETN, and that you should call external C routines with near CALL instructions.

• In models using more than one code segment (medium, large and huge), functions are far. Thismeans that function pointers are 32 bits long (consisting of a 16−bit offset followed by a 16−bitsegment), and that functions are called using CALL FAR (or CALL seg:offset ) and returnusing RETF. Again, you should therefore write your own routines to return with RETF and useCALL FAR to call external routines.

• In models using a single data segment (tiny, small and medium), data pointers are 16 bits long,containing only an offset field (the DS register doesn’t change its value, and always gives thesegment part of the full data item address).

• In models using more than one data segment (compact, large and huge), data pointers are 32 bitslong, consisting of a 16−bit offset followed by a 16−bit segment. You should still be careful notto modify DS in your routines without restoring it afterwards, but ES is free for you to use toaccess the contents of 32−bit data pointers you are passed.

• The huge memory model allows single data items to exceed 64K in size. In all other memorymodels, you can access the whole of a data item just by doing arithmetic on the offset field of thepointer you are given, whether a segment field is present or not; in huge model, you have to bemore careful of your pointer arithmetic.

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• In most memory models, there is a default data segment, whose segment address is kept in DSthroughout the program. This data segment is typically the same segment as the stack, kept in SS,so that functions’ local variables (which are stored on the stack) and global data items can bothbe accessed easily without changing DS. Particularly large data items are typically stored in othersegments. However, some memory models (though not the standard ones, usually) allow theassumption that SS and DS hold the same value to be removed. Be careful about functions’ localvariables in this latter case.

In models with a single code segment, the segment is called _TEXT, so your code segment mustalso go by this name in order to be linked into the same place as the main code segment. In modelswith a single data segment, or with a default data segment, it is called _DATA.

7.4.3 Function Definitions and Function CallsThe C calling convention in 16−bit programs is as follows. In the following description, the wordscaller and callee are used to denote the function doing the calling and the function which gets called.

• The caller pushes the function’s parameters on the stack, one after another, in reverse order (rightto left, so that the first argument specified to the function is pushed last).

• The caller then executes a CALL instruction to pass control to the callee. This CALL is either nearor far depending on the memory model.

• The callee receives control, and typically (although this is not actually necessary, in functionswhich do not need to access their parameters) starts by saving the value of SP in BP so as to beable to use BP as a base pointer to find its parameters on the stack. However, the caller wasprobably doing this too, so part of the calling convention states that BP must be preserved by anyC function. Hence the callee, if it is going to set up BP as a frame pointer, must push the previousvalue first.

• The callee may then access its parameters relative to BP. The word at [BP] holds the previousvalue of BP as it was pushed; the next word, at [BP+2] , holds the offset part of the returnaddress, pushed implicitly by CALL. In a small−model (near) function, the parameters start afterthat, at [BP+4] ; in a large−model (far) function, the segment part of the return address lives at[BP+4] , and the parameters begin at [BP+6] . The leftmost parameter of the function, since itwas pushed last, is accessible at this offset from BP; the others follow, at successively greateroffsets. Thus, in a function such as printf which takes a variable number of parameters, thepushing of the parameters in reverse order means that the function knows where to find its firstparameter, which tells it the number and type of the remaining ones.

• The callee may also wish to decrease SP further, so as to allocate space on the stack for localvariables, which will then be accessible at negative offsets from BP.

• The callee, if it wishes to return a value to the caller, should leave the value in AL, AX or DX:AXdepending on the size of the value. Floating−point results are sometimes (depending on thecompiler) returned in ST0.

• Once the callee has finished processing, it restores SP from BP if it had allocated local stackspace, then pops the previous value of BP, and returns via RETN or RETF depending on memorymodel.

• When the caller regains control from the callee, the function parameters are still on the stack, soit typically adds an immediate constant to SP to remove them (instead of executing a number ofslow POP instructions). Thus, if a function is accidentally called with the wrong number ofparameters due to a prototype mismatch, the stack will still be returned to a sensible state sincethe caller, which knows how many parameters it pushed, does the removing.

It is instructive to compare this calling convention with that for Pascal programs (described insection 7.5.1). Pascal has a simpler convention, since no functions have variable numbers of

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parameters. Therefore the callee knows how many parameters it should have been passed, and isable to deallocate them from the stack itself by passing an immediate argument to the RET or RETFinstruction, so the caller does not have to do it. Also, the parameters are pushed in left−to−rightorder, not right−to−left, which means that a compiler can give better guarantees about sequencepoints without performance suffering.

Thus, you would define a function in C style in the following way. The following example is forsmall model:

global _myfunc

_myfunc: push bp mov bp,sp sub sp,0x40 ; 64 bytes of local stack space mov bx,[bp+4] ; first parameter to function

; some more code

mov sp,bp ; undo "sub sp,0x40" above pop bp ret

For a large−model function, you would replace RET by RETF, and look for the first parameter at[BP+6] instead of [BP+4] . Of course, if one of the parameters is a pointer, then the offsets ofsubsequent parameters will change depending on the memory model as well: far pointers take upfour bytes on the stack when passed as a parameter, whereas near pointers take up two.

At the other end of the process, to call a C function from your assembly code, you would dosomething like this:

extern _printf

; and then, further down...

push word [myint] ; one of my integer variables push word mystring ; pointer into my data segment call _printf add sp,byte 4 ; ‘byte’ saves space

; then those data items...

segment _DATA

myint dw 1234 mystring db ’This number −> %d <− should be 1234’,10,0

This piece of code is the small−model assembly equivalent of the C code

int myint = 1234; printf("This number −> %d <− should be 1234\\n", myint);

In large model, the function−call code might look more like this. In this example, it is assumed thatDS already holds the segment base of the segment _DATA. If not, you would have to initialise itfirst.

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push word [myint] push word seg mystring ; Now push the segment, and... push word mystring ; ... offset of "mystring" call far _printf add sp,byte 6

The integer value still takes up one word on the stack, since large model does not affect the size ofthe int data type. The first argument (pushed last) to printf , however, is a data pointer, andtherefore has to contain a segment and offset part. The segment should be stored second in memory,and therefore must be pushed first. (Of course, PUSH DS would have been a shorter instructionthan PUSH WORD SEG mystring , if DS was set up as the above example assumed.) Then theactual call becomes a far call, since functions expect far calls in large model; and SP has to beincreased by 6 rather than 4 afterwards to make up for the extra word of parameters.

7.4.4 Accessing Data ItemsTo get at the contents of C variables, or to declare variables which C can access, you need onlydeclare the names as GLOBAL or EXTERN. (Again, the names require leading underscores, as statedin section 7.4.1.) Thus, a C variable declared as int i can be accessed from assembler as

extern _i

mov ax,[_i]

And to declare your own integer variable which C programs can access as extern int j , youdo this (making sure you are assembling in the _DATA segment, if necessary):

global _j

_j dw 0

To access a C array, you need to know the size of the components of the array. For example, intvariables are two bytes long, so if a C program declares an array as int a[10] , you can accessa[3] by coding mov ax,[_a+6] . (The byte offset 6 is obtained by multiplying the desired arrayindex, 3, by the size of the array element, 2.) The sizes of the C base types in 16−bit compilers are:1 for char , 2 for short and int , 4 for long and float , and 8 for double .

To access a C data structure, you need to know the offset from the base of the structure to the fieldyou are interested in. You can either do this by converting the C structure definition into a NASMstructure definition (using STRUC), or by calculating the one offset and using just that.

To do either of these, you should read your C compiler’s manual to find out how it organises datastructures. NASM gives no special alignment to structure members in its own STRUC macro, so youhave to specify alignment yourself if the C compiler generates it. Typically, you might find that astructure like

struct @\{ char c; int i; @\} foo;

might be four bytes long rather than three, since the int field would be aligned to a two−byteboundary. However, this sort of feature tends to be a configurable option in the C compiler, eitherusing command−line options or #pragma lines, so you have to find out how your own compilerdoes it.

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7.4.5 c16.mac : Helper Macros for the 16−bit C InterfaceIncluded in the NASM archives, in the misc directory, is a file c16.mac of macros. It definesthree macros: proc , arg and endproc . These are intended to be used for C−style proceduredefinitions, and they automate a lot of the work involved in keeping track of the calling convention.

(An alternative, TASM compatible form of arg is also now built into NASM’s preprocessor. Seesection 4.9 for details.)

An example of an assembly function using the macro set is given here:

proc _nearproc

%$i arg %$j arg mov ax,[bp + %$i] mov bx,[bp + %$j] add ax,[bx]

endproc

This defines _nearproc to be a procedure taking two arguments, the first (i ) an integer and thesecond (j ) a pointer to an integer. It returns i + *j .

Note that the arg macro has an EQU as the first line of its expansion, and since the label before themacro call gets prepended to the first line of the expanded macro, the EQU works, defining %$i tobe an offset from BP. A context−local variable is used, local to the context pushed by the procmacro and popped by the endproc macro, so that the same argument name can be used in laterprocedures. Of course, you don’t have to do that.

The macro set produces code for near functions (tiny, small and compact−model code) by default.You can have it generate far functions (medium, large and huge−model code) by means of coding%define FARCODE . This changes the kind of return instruction generated by endproc , and alsochanges the starting point for the argument offsets. The macro set contains no intrinsic dependencyon whether data pointers are far or not.

arg can take an optional parameter, giving the size of the argument. If no size is given, 2 isassumed, since it is likely that many function parameters will be of type int .

The large−model equivalent of the above function would look like this:

%define FARCODE

proc _farproc

%$i arg %$j arg 4 mov ax,[bp + %$i] mov bx,[bp + %$j] mov es,[bp + %$j + 2] add ax,[bx]

endproc

This makes use of the argument to the arg macro to define a parameter of size 4, because j is nowa far pointer. When we load from j , we must load a segment and an offset.

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7.5 Interfacing to Borland Pascal ProgramsInterfacing to Borland Pascal programs is similar in concept to interfacing to 16−bit C programs.The differences are:

• The leading underscore required for interfacing to C programs is not required for Pascal.

• The memory model is always large: functions are far, data pointers are far, and no data item canbe more than 64K long. (Actually, some functions are near, but only those functions that are localto a Pascal unit and never called from outside it. All assembly functions that Pascal calls, and allPascal functions that assembly routines are able to call, are far.) However, all static data declaredin a Pascal program goes into the default data segment, which is the one whose segment addresswill be in DS when control is passed to your assembly code. The only things that do not live inthe default data segment are local variables (they live in the stack segment) and dynamicallyallocated variables. All data pointers, however, are far.

• The function calling convention is different – described below.

• Some data types, such as strings, are stored differently.

• There are restrictions on the segment names you are allowed to use – Borland Pascal will ignorecode or data declared in a segment it doesn’t like the name of. The restrictions are describedbelow.

7.5.1 The Pascal Calling ConventionThe 16−bit Pascal calling convention is as follows. In the following description, the words callerand callee are used to denote the function doing the calling and the function which gets called.

• The caller pushes the function’s parameters on the stack, one after another, in normal order (leftto right, so that the first argument specified to the function is pushed first).

• The caller then executes a far CALL instruction to pass control to the callee.

• The callee receives control, and typically (although this is not actually necessary, in functionswhich do not need to access their parameters) starts by saving the value of SP in BP so as to beable to use BP as a base pointer to find its parameters on the stack. However, the caller wasprobably doing this too, so part of the calling convention states that BP must be preserved by anyfunction. Hence the callee, if it is going to set up BP as a frame pointer, must push the previousvalue first.

• The callee may then access its parameters relative to BP. The word at [BP] holds the previousvalue of BP as it was pushed. The next word, at [BP+2] , holds the offset part of the returnaddress, and the next one at [BP+4] the segment part. The parameters begin at [BP+6] . Therightmost parameter of the function, since it was pushed last, is accessible at this offset from BP;the others follow, at successively greater offsets.

• The callee may also wish to decrease SP further, so as to allocate space on the stack for localvariables, which will then be accessible at negative offsets from BP.

• The callee, if it wishes to return a value to the caller, should leave the value in AL, AX or DX:AXdepending on the size of the value. Floating−point results are returned in ST0. Results of typeReal (Borland’s own custom floating−point data type, not handled directly by the FPU) arereturned in DX:BX:AX . To return a result of type String , the caller pushes a pointer to atemporary string before pushing the parameters, and the callee places the returned string value atthat location. The pointer is not a parameter, and should not be removed from the stack by theRETF instruction.

• Once the callee has finished processing, it restores SP from BP if it had allocated local stackspace, then pops the previous value of BP, and returns via RETF. It uses the form of RETF with

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an immediate parameter, giving the number of bytes taken up by the parameters on the stack.This causes the parameters to be removed from the stack as a side effect of the return instruction.

• When the caller regains control from the callee, the function parameters have already beenremoved from the stack, so it needs to do nothing further.

Thus, you would define a function in Pascal style, taking two Integer –type parameters, in thefollowing way:

global myfunc

myfunc: push bp mov bp,sp sub sp,0x40 ; 64 bytes of local stack space mov bx,[bp+8] ; first parameter to function mov bx,[bp+6] ; second parameter to function

; some more code

mov sp,bp ; undo "sub sp,0x40" above pop bp retf 4 ; total size of params is 4

At the other end of the process, to call a Pascal function from your assembly code, you would dosomething like this:

extern SomeFunc


push word seg mystring ; Now push the segment, and... push word mystring ; ... offset of "mystring" push word [myint] ; one of my variables call far SomeFunc

This is equivalent to the Pascal code

procedure SomeFunc(String: PChar; Int: Integer); SomeFunc(@@mystring, myint);

7.5.2 Borland Pascal Segment Name RestrictionsSince Borland Pascal’s internal unit file format is completely different from OBJ, it only makes avery sketchy job of actually reading and understanding the various information contained in a realOBJ file when it links that in. Therefore an object file intended to be linked to a Pascal programmust obey a number of restrictions:

• Procedures and functions must be in a segment whose name is either CODE, CSEG, or somethingending in _TEXT.

• Initialised data must be in a segment whose name is either CONST or something ending in_DATA.

• Uninitialised data must be in a segment whose name is either DATA, DSEG, or something endingin _BSS.

• Any other segments in the object file are completely ignored. GROUP directives and segmentattributes are also ignored.

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7.5.3 Using c16.mac With Pascal ProgramsThe c16.mac macro package, described in section 7.4.5, can also be used to simplify writingfunctions to be called from Pascal programs, if you code %define PASCAL . This definitionensures that functions are far (it implies FARCODE), and also causes procedure return instructions tobe generated with an operand.

Defining PASCAL does not change the code which calculates the argument offsets; you mustdeclare your function’s arguments in reverse order. For example:

%define PASCAL

proc _pascalproc

%$j arg 4 %$i arg mov ax,[bp + %$i] mov bx,[bp + %$j] mov es,[bp + %$j + 2] add ax,[bx]

endproc

This defines the same routine, conceptually, as the example in section 7.4.5: it defines a functiontaking two arguments, an integer and a pointer to an integer, which returns the sum of the integerand the contents of the pointer. The only difference between this code and the large−model Cversion is that PASCAL is defined instead of FARCODE, and that the arguments are declared inreverse order.

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Chapter 8: Writing 32−bit Code (Unix, Win32, DJGPP)

This chapter attempts to cover some of the common issues involved when writing 32−bit code, torun under Win32 or Unix, or to be linked with C code generated by a Unix−style C compiler suchas DJGPP. It covers how to write assembly code to interface with 32−bit C routines, and how towrite position−independent code for shared libraries.

Almost all 32−bit code, and in particular all code running under Win32 , DJGPP or any of the PCUnix variants, runs in flat memory model. This means that the segment registers and paging havealready been set up to give you the same 32−bit 4Gb address space no matter what segment youwork relative to, and that you should ignore all segment registers completely. When writingflat−model application code, you never need to use a segment override or modify any segmentregister, and the code−section addresses you pass to CALL and JMP live in the same address spaceas the data−section addresses you access your variables by and the stack−section addresses youaccess local variables and procedure parameters by. Every address is 32 bits long and contains onlyan offset part.

8.1 Interfacing to 32−bit C ProgramsA lot of the discussion in section 7.4, about interfacing to 16−bit C programs, still applies whenworking in 32 bits. The absence of memory models or segmentation worries simplifies things a lot.

8.1.1 External Symbol NamesMost 32−bit C compilers share the convention used by 16−bit compilers, that the names of allglobal symbols (functions or data) they define are formed by prefixing an underscore to the name asit appears in the C program. However, not all of them do: the ELF specification states that Csymbols do not have a leading underscore on their assembly−language names.

The older Linux a.out C compiler, all Win32 compilers, DJGPP, and NetBSD and FreeBSD ,all use the leading underscore; for these compilers, the macros cextern and cglobal , as givenin section 7.4.1, will still work. For ELF, though, the leading underscore should not be used.

8.1.2 Function Definitions and Function CallsThe C calling conventionThe C calling convention in 32−bit programs is as follows. In thefollowing description, the words caller and callee are used to denote the function doing the callingand the function which gets called.

• The caller pushes the function’s parameters on the stack, one after another, in reverse order (rightto left, so that the first argument specified to the function is pushed last).

• The caller then executes a near CALL instruction to pass control to the callee.

• The callee receives control, and typically (although this is not actually necessary, in functionswhich do not need to access their parameters) starts by saving the value of ESP in EBP so as tobe able to use EBP as a base pointer to find its parameters on the stack. However, the caller wasprobably doing this too, so part of the calling convention states that EBP must be preserved byany C function. Hence the callee, if it is going to set up EBP as a frame pointer, must push theprevious value first.

• The callee may then access its parameters relative to EBP. The doubleword at [EBP] holds theprevious value of EBP as it was pushed; the next doubleword, at [EBP+4] , holds the returnaddress, pushed implicitly by CALL. The parameters start after that, at [EBP+8] . The leftmost

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parameter of the function, since it was pushed last, is accessible at this offset from EBP; theothers follow, at successively greater offsets. Thus, in a function such as printf which takes avariable number of parameters, the pushing of the parameters in reverse order means that thefunction knows where to find its first parameter, which tells it the number and type of theremaining ones.

• The callee may also wish to decrease ESP further, so as to allocate space on the stack for localvariables, which will then be accessible at negative offsets from EBP.

• The callee, if it wishes to return a value to the caller, should leave the value in AL, AX or EAXdepending on the size of the value. Floating−point results are typically returned in ST0.

• Once the callee has finished processing, it restores ESP from EBP if it had allocated local stackspace, then pops the previous value of EBP, and returns via RET (equivalently, RETN).

• When the caller regains control from the callee, the function parameters are still on the stack, soit typically adds an immediate constant to ESP to remove them (instead of executing a number ofslow POP instructions). Thus, if a function is accidentally called with the wrong number ofparameters due to a prototype mismatch, the stack will still be returned to a sensible state sincethe caller, which knows how many parameters it pushed, does the removing.

There is an alternative calling convention used by Win32 programs for Windows API calls, and alsofor functions called by the Windows API such as window procedures: they follow what Microsoftcalls the __stdcall convention. This is slightly closer to the Pascal convention, in that the calleeclears the stack by passing a parameter to the RET instruction. However, the parameters are stillpushed in right−to−left order.

Thus, you would define a function in C style in the following way:

global _myfunc

_myfunc: push ebp mov ebp,esp sub esp,0x40 ; 64 bytes of local stack space mov ebx,[ebp+8] ; first parameter to function

; some more code

leave ; mov esp,ebp / pop ebp ret

At the other end of the process, to call a C function from your assembly code, you would dosomething like this:

extern _printf


push dword [myint] ; one of my integer variables push dword mystring ; pointer into my data segment call _printf add esp,byte 8 ; ‘byte’ saves space

; then those data items...

segment _DATA

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myint dd 1234 mystring db ’This number −> %d <− should be 1234’,10,0

This piece of code is the assembly equivalent of the C code

int myint = 1234; printf("This number −> %d <− should be 1234\\n", myint);

8.1.3 Accessing Data ItemsTo get at the contents of C variables, or to declare variables which C can access, you need onlydeclare the names as GLOBAL or EXTERN. (Again, the names require leading underscores, as statedin section 8.1.1.) Thus, a C variable declared as int i can be accessed from assembler as

extern _i mov eax,[_i]

And to declare your own integer variable which C programs can access as extern int j , youdo this (making sure you are assembling in the _DATA segment, if necessary):

global _j _j dd 0

To access a C array, you need to know the size of the components of the array. For example, intvariables are four bytes long, so if a C program declares an array as int a[10] , you can accessa[3] by coding mov ax,[_a+12] . (The byte offset 12 is obtained by multiplying the desiredarray index, 3, by the size of the array element, 4.) The sizes of the C base types in 32−bit compilersare: 1 for char , 2 for short , 4 for int , long and float , and 8 for double . Pointers, being32−bit addresses, are also 4 bytes long.

To access a C data structure, you need to know the offset from the base of the structure to the fieldyou are interested in. You can either do this by converting the C structure definition into a NASMstructure definition (using STRUC), or by calculating the one offset and using just that.

To do either of these, you should read your C compiler’s manual to find out how it organises datastructures. NASM gives no special alignment to structure members in its own STRUC macro, so youhave to specify alignment yourself if the C compiler generates it. Typically, you might find that astructure like

struct @\{ char c; int i; @\} foo;

might be eight bytes long rather than five, since the int field would be aligned to a four−byteboundary. However, this sort of feature is sometimes a configurable option in the C compiler, eitherusing command−line options or #pragma lines, so you have to find out how your own compilerdoes it.

8.1.4 c32.mac : Helper Macros for the 32−bit C InterfaceIncluded in the NASM archives, in the misc directory, is a file c32.mac of macros. It definesthree macros: proc , arg and endproc . These are intended to be used for C−style proceduredefinitions, and they automate a lot of the work involved in keeping track of the calling convention.

An example of an assembly function using the macro set is given here:

proc _proc32

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%$i arg %$j arg mov eax,[ebp + %$i] mov ebx,[ebp + %$j] add eax,[ebx]

endproc

This defines _proc32 to be a procedure taking two arguments, the first (i ) an integer and thesecond (j ) a pointer to an integer. It returns i + *j .

Note that the arg macro has an EQU as the first line of its expansion, and since the label before themacro call gets prepended to the first line of the expanded macro, the EQU works, defining %$i tobe an offset from BP. A context−local variable is used, local to the context pushed by the procmacro and popped by the endproc macro, so that the same argument name can be used in laterprocedures. Of course, you don’t have to do that.

arg can take an optional parameter, giving the size of the argument. If no size is given, 4 isassumed, since it is likely that many function parameters will be of type int or pointers.

8.2 Writing NetBSD/FreeBSD/OpenBSD and Linux/ELF Shared LibrariesELF replaced the older a.out object file format under Linux because it contains support forposition−independent code (PIC), which makes writing shared libraries much easier. NASMsupports the ELF position−independent code features, so you can write Linux ELF shared librariesin NASM.

NetBSD, and its close cousins FreeBSD and OpenBSD, take a different approach by hacking PICsupport into the a.out format. NASM supports this as the aoutb output format, so you can writeBSD shared libraries in NASM too.

The operating system loads a PIC shared library by memory−mapping the library file at anarbitrarily chosen point in the address space of the running process. The contents of the library’scode section must therefore not depend on where it is loaded in memory.

Therefore, you cannot get at your variables by writing code like this:

mov eax,[myvar] ; WRONG

Instead, the linker provides an area of memory called the global offset table, or GOT; the GOT issituated at a constant distance from your library’s code, so if you can find out where your library isloaded (which is typically done using a CALL and POP combination), you can obtain the address ofthe GOT, and you can then load the addresses of your variables out of linker−generated entries inthe GOT.

The data section of a PIC shared library does not have these restrictions: since the data section iswritable, it has to be copied into memory anyway rather than just paged in from the library file, soas long as it’s being copied it can be relocated too. So you can put ordinary types of relocation inthe data section without too much worry (but see section 8.2.4 for a caveat).

8.2.1 Obtaining the Address of the GOTEach code module in your shared library should define the GOT as an external symbol:

extern _GLOBAL_OFFSET_TABLE_ ; in ELF extern __GLOBAL_OFFSET_TABLE_ ; in BSD a.out

At the beginning of any function in your shared library which plans to access your data or BSSsections, you must first calculate the address of the GOT. This is typically done by writing thefunction in this form:

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func: push ebp mov ebp,esp push ebx call .get_GOT .get_GOT: pop ebx add ebx,_GLOBAL_OFFSET_TABLE_+$$−.get_GOT wrt ..gotpc

; the function body comes here

mov ebx,[ebp−4] mov esp,ebp pop ebp ret

(For BSD, again, the symbol _GLOBAL_OFFSET_TABLE requires a second leading underscore.)

The first two lines of this function are simply the standard C prologue to set up a stack frame, andthe last three lines are standard C function epilogue. The third line, and the fourth to last line, saveand restore the EBX register, because PIC shared libraries use this register to store the address of theGOT.

The interesting bit is the CALL instruction and the following two lines. The CALL and POPcombination obtains the address of the label .get_GOT , without having to know in advance wherethe program was loaded (since the CALL instruction is encoded relative to the current position). TheADD instruction makes use of one of the special PIC relocation types: GOTPC relocation. With theWRT ..gotpc qualifier specified, the symbol referenced (here _GLOBAL_OFFSET_TABLE_,the special symbol assigned to the GOT) is given as an offset from the beginning of the section.(Actually, ELF encodes it as the offset from the operand field of the ADD instruction, but NASMsimplifies this deliberately, so you do things the same way for both ELF and BSD.) So theinstruction then adds the beginning of the section, to get the real address of the GOT, and subtractsthe value of .get_GOT which it knows is in EBX. Therefore, by the time that instruction hasfinished, EBX contains the address of the GOT.

If you didn’t follow that, don’t worry: it’s never necessary to obtain the address of the GOT by anyother means, so you can put those three instructions into a macro and safely ignore them:

%macro get_GOT 0

call %%getgot %%getgot: pop ebx add ebx,_GLOBAL_OFFSET_TABLE_+$$−%%getgot wrt ..gotpc

%endmacro

8.2.2 Finding Your Local Data ItemsHaving got the GOT, you can then use it to obtain the addresses of your data items. Most variableswill reside in the sections you have declared; they can be accessed using the ..gotoff specialWRT type. The way this works is like this:

lea eax,[ebx+myvar wrt ..gotoff]

The expression myvar wrt ..gotoff is calculated, when the shared library is linked, to be theoffset to the local variable myvar from the beginning of the GOT. Therefore, adding it to EBX asabove will place the real address of myvar in EAX.

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If you declare variables as GLOBAL without specifying a size for them, they are shared betweencode modules in the library, but do not get exported from the library to the program that loaded it.They will still be in your ordinary data and BSS sections, so you can access them in the same wayas local variables, using the above ..gotoff mechanism.

Note that due to a peculiarity of the way BSD a.out format handles this relocation type, theremust be at least one non−local symbol in the same section as the address you’re trying to access.

8.2.3 Finding External and Common Data ItemsIf your library needs to get at an external variable (external to the library, not just to one of themodules within it), you must use the ..got type to get at it. The ..got type, instead of givingyou the offset from the GOT base to the variable, gives you the offset from the GOT base to a GOTentry containing the address of the variable. The linker will set up this GOT entry when it builds thelibrary, and the dynamic linker will place the correct address in it at load time. So to obtain theaddress of an external variable extvar in EAX, you would code

mov eax,[ebx+extvar wrt ..got]

This loads the address of extvar out of an entry in the GOT. The linker, when it builds the sharedlibrary, collects together every relocation of type ..got , and builds the GOT so as to ensure it hasevery necessary entry present.

Common variables must also be accessed in this way.

8.2.4 Exporting Symbols to the Library UserIf you want to export symbols to the user of the library, you have to declare whether they arefunctions or data, and if they are data, you have to give the size of the data item. This is because thedynamic linker has to build procedure linkage table entries for any exported functions, and alsomoves exported data items away from the library’s data section in which they were declared.

So to export a function to users of the library, you must use

global func:function ; declare it as a function

func: push ebp

; etc.

And to export a data item such as an array, you would have to code

global array:data array.end−array ; give the size too

array: resd 128 .end:

Be careful: If you export a variable to the library user, by declaring it as GLOBAL and supplying asize, the variable will end up living in the data section of the main program, rather than in yourlibrary’s data section, where you declared it. So you will have to access your own global variablewith the ..got mechanism rather than ..gotoff , as if it were external (which, effectively, it hasbecome).

Equally, if you need to store the address of an exported global in one of your data sections, youcan’t do it by means of the standard sort of code:

dataptr: dd global_data_item ; WRONG

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NASM will interpret this code as an ordinary relocation, in which global_data_item is merelyan offset from the beginning of the .data section (or whatever); so this reference will end uppointing at your data section instead of at the exported global which resides elsewhere.

Instead of the above code, then, you must write

dataptr: dd global_data_item wrt ..sym

which makes use of the special WRT type ..sym to instruct NASM to search the symbol table for aparticular symbol at that address, rather than just relocating by section base.

Either method will work for functions: referring to one of your functions by means of

funcptr: dd my_function

will give the user the address of the code you wrote, whereas

funcptr: dd my_function wrt .sym

will give the address of the procedure linkage table for the function, which is where the callingprogram will believe the function lives. Either address is a valid way to call the function.

8.2.5 Calling Procedures Outside the LibraryCalling procedures outside your shared library has to be done by means of a procedure linkagetable, or PLT. The PLT is placed at a known offset from where the library is loaded, so the librarycode can make calls to the PLT in a position−independent way. Within the PLT there is code tojump to offsets contained in the GOT, so function calls to other shared libraries or to routines in themain program can be transparently passed off to their real destinations.

To call an external routine, you must use another special PIC relocation type, WRT ..plt . This ismuch easier than the GOT−based ones: you simply replace calls such as CALL printf with thePLT−relative version CALL printf WRT ..plt .

8.2.6 Generating the Library FileHaving written some code modules and assembled them to .o files, you then generate your sharedlibrary with a command such as

ld −shared −o library.so module1.o module2.o # for ELF ld −Bshareable −o library.so module1.o module2.o # for BSD

For ELF, if your shared library is going to reside in system directories such as /usr/lib or/lib , it is usually worth using the −soname flag to the linker, to store the final library file name,with a version number, into the library:

ld −shared −soname library.so.1 −o library.so.1.2 *.o

You would then copy library.so.1.2 into the library directory, and create library.so.1as a symbolic link to it.

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Chapter 9: Mixing 16 and 32 Bit Code

This chapter tries to cover some of the issues, largely related to unusual forms of addressing andjump instructions, encountered when writing operating system code such as protected−modeinitialisation routines, which require code that operates in mixed segment sizes, such as code in a16−bit segment trying to modify data in a 32−bit one, or jumps between different−size segments.

9.1 Mixed−Size JumpsThe most common form of mixed−size instruction is the one used when writing a 32−bit OS:having done your setup in 16−bit mode, such as loading the kernel, you then have to boot it byswitching into protected mode and jumping to the 32−bit kernel start address. In a fully 32−bit OS,this tends to be the only mixed−size instruction you need, since everything before it can be done inpure 16−bit code, and everything after it can be pure 32−bit.

This jump must specify a 48−bit far address, since the target segment is a 32−bit one. However, itmust be assembled in a 16−bit segment, so just coding, for example,

jmp 0x1234:0x56789ABC ; wrong!

will not work, since the offset part of the address will be truncated to 0x9ABC and the jump will bean ordinary 16−bit far one.

The Linux kernel setup code gets round the inability of as86 to generate the required instruction bycoding it manually, using DB instructions. NASM can go one better than that, by actually generatingthe right instruction itself. Here’s how to do it right:

jmp dword 0x1234:0x56789ABC ; right

The DWORD prefix (strictly speaking, it should come after the colon, since it is declaring the offsetfield to be a doubleword; but NASM will accept either form, since both are unambiguous) forcesthe offset part to be treated as far, in the assumption that you are deliberately writing a jump from a16−bit segment to a 32−bit one.

You can do the reverse operation, jumping from a 32−bit segment to a 16−bit one, by means of theWORD prefix:

jmp word 0x8765:0x4321 ; 32 to 16 bit

If the WORD prefix is specified in 16−bit mode, or the DWORD prefix in 32−bit mode, they will beignored, since each is explicitly forcing NASM into a mode it was in anyway.

9.2 Addressing Between Different−Size SegmentsIf your OS is mixed 16 and 32−bit, or if you are writing a DOS extender, you are likely to have todeal with some 16−bit segments and some 32−bit ones. At some point, you will probably end upwriting code in a 16−bit segment which has to access data in a 32−bit segment, or vice versa.

If the data you are trying to access in a 32−bit segment lies within the first 64K of the segment, youmay be able to get away with using an ordinary 16−bit addressing operation for the purpose; butsooner or later, you will want to do 32−bit addressing from 16−bit mode.

The easiest way to do this is to make sure you use a register for the address, since any effectiveaddress containing a 32−bit register is forced to be a 32−bit address. So you can do

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mov eax,offset_into_32_bit_segment_specified_by_fs mov dword [fs:eax],0x11223344

This is fine, but slightly cumbersome (since it wastes an instruction and a register) if you alreadyknow the precise offset you are aiming at. The x86 architecture does allow 32−bit effectiveaddresses to specify nothing but a 4−byte offset, so why shouldn’t NASM be able to generate thebest instruction for the purpose?

It can. As in section 9.1, you need only prefix the address with the DWORD keyword, and it will beforced to be a 32−bit address:

mov dword [fs:dword my_offset],0x11223344

Also as in section 9.1, NASM is not fussy about whether the DWORD prefix comes before or afterthe segment override, so arguably a nicer−looking way to code the above instruction is

mov dword [dword fs:my_offset],0x11223344

Don’t confuse the DWORD prefix outside the square brackets, which controls the size of the datastored at the address, with the one inside the square brackets which controls the length of theaddress itself. The two can quite easily be different:

mov word [dword 0x12345678],0x9ABC

This moves 16 bits of data to an address specified by a 32−bit offset.

You can also specify WORD or DWORD prefixes along with the FAR prefix to indirect far jumps orcalls. For example:

call dword far [fs:word 0x4321]

This instruction contains an address specified by a 16−bit offset; it loads a 48−bit far pointer fromthat (16−bit segment and 32−bit offset), and calls that address.

9.3 Other Mixed−Size InstructionsThe other way you might want to access data might be using the string instructions (LODSx,STOSx and so on) or the XLATB instruction. These instructions, since they take no parameters,might seem to have no easy way to make them perform 32−bit addressing when assembled in a16−bit segment.

This is the purpose of NASM’s a16 and a32 prefixes. If you are coding LODSB in a 16−bitsegment but it is supposed to be accessing a string in a 32−bit segment, you should load the desiredaddress into ESI and then code

a32 lodsb

The prefix forces the addressing size to 32 bits, meaning that LODSB loads from [DS:ESI]instead of [DS:SI] . To access a string in a 16−bit segment when coding in a 32−bit one, thecorresponding a16 prefix can be used.

The a16 and a32 prefixes can be applied to any instruction in NASM’s instruction table, but mostof them can generate all the useful forms without them. The prefixes are necessary only forinstructions with implicit addressing: CMPSx (section B.4.27), SCASx (section B.4.286), LODSx(section B.4.141), STOSx (section B.4.303), MOVSx (section B.4.178), INSx (section B.4.121),OUTSx (section B.4.195), and XLATB (section B.4.334). Also, the various push and popinstructions (PUSHA and POPF as well as the more usual PUSH and POP) can accept a16 or a32prefixes to force a particular one of SP or ESP to be used as a stack pointer, in case the stacksegment in use is a different size from the code segment.

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PUSH and POP, when applied to segment registers in 32−bit mode, also have the slightly oddbehaviour that they push and pop 4 bytes at a time, of which the top two are ignored and the bottomtwo give the value of the segment register being manipulated. To force the 16−bit behaviour ofsegment−register push and pop instructions, you can use the operand−size prefix o16 :

o16 push ss o16 push ds

This code saves a doubleword of stack space by fitting two segment registers into the space whichwould normally be consumed by pushing one.

(You can also use the o32 prefix to force the 32−bit behaviour when in 16−bit mode, but this seemsless useful.)

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Chapter 10: Troubleshooting

This chapter describes some of the common problems that users have been known to encounter withNASM, and answers them. It also gives instructions for reporting bugs in NASM if you find adifficulty that isn’t listed here.

10.1 Common Problems10.1.1 NASM Generates Inefficient Code

I get a lot of ‘bug’ reports about NASM generating inefficient, or even ‘wrong’, code oninstructions such as ADD ESP,8 . This is a deliberate design feature, connected to predictability ofoutput: NASM, on seeing ADD ESP,8 , will generate the form of the instruction which leaves roomfor a 32−bit offset. You need to code ADD ESP,BYTE 8 if you want the space−efficient form ofthe instruction. This isn’t a bug: at worst it’s a misfeature, and that’s a matter of opinion only.

10.1.2 My Jumps are Out of RangeSimilarly, people complain that when they issue conditional jumps (which are SHORT by default)that try to jump too far, NASM reports ‘short jump out of range’ instead of making the jumps longer.

This, again, is partly a predictability issue, but in fact has a more practical reason as well. NASMhas no means of being told what type of processor the code it is generating will be run on; so itcannot decide for itself that it should generate Jcc NEAR type instructions, because it doesn’tknow that it’s working for a 386 or above. Alternatively, it could replace the out−of−range shortJNE instruction with a very short JE instruction that jumps over a JMP NEAR; this is a sensiblesolution for processors below a 386, but hardly efficient on processors which have good branchprediction and could have used JNE NEAR instead. So, once again, it’s up to the user, not theassembler, to decide what instructions should be generated.

10.1.3 ORG Doesn’t WorkPeople writing boot sector programs in the bin format often complain that ORG doesn’t work theway they’d like: in order to place the 0xAA55 signature word at the end of a 512−byte boot sector,people who are used to MASM tend to code

ORG 0

; some boot sector code

ORG 510 DW 0xAA55

This is not the intended use of the ORG directive in NASM, and will not work. The correct way tosolve this problem in NASM is to use the TIMES directive, like this:

ORG 0

; some boot sector code

TIMES 510−($−$$) DB 0 DW 0xAA55

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The TIMES directive will insert exactly enough zero bytes into the output to move the assemblypoint up to 510. This method also has the advantage that if you accidentally fill your boot sector toofull, NASM will catch the problem at assembly time and report it, so you won’t end up with a bootsector that you have to disassemble to find out what’s wrong with it.

10.1.4 TIMES Doesn’t WorkThe other common problem with the above code is people who write the TIMES line as

TIMES 510−$ DB 0

by reasoning that $ should be a pure number, just like 510, so the difference between them is also apure number and can happily be fed to TIMES.

NASM is a modular assembler: the various component parts are designed to be easily separable forre−use, so they don’t exchange information unnecessarily. In consequence, the bin output format,even though it has been told by the ORG directive that the .text section should start at 0, does notpass that information back to the expression evaluator. So from the evaluator’s point of view, $isn’t a pure number: it’s an offset from a section base. Therefore the difference between $ and 510is also not a pure number, but involves a section base. Values involving section bases cannot bepassed as arguments to TIMES.

The solution, as in the previous section, is to code the TIMES line in the form

TIMES 510−($−$$) DB 0

in which $ and $$ are offsets from the same section base, and so their difference is a pure number.This will solve the problem and generate sensible code.

10.2 BugsWe have never yet released a version of NASM with any known bugs. That doesn’t usually stopthere being plenty we didn’t know about, though. Any that you find should be reported firstly viathe bugtracker at https://sourceforge.net/projects/nasm/ (click on "Bugs"), orif that fails then through one of the contacts in section 1.2.

Please read section 2.2 first, and don’t report the bug if it’s listed in there as a deliberate feature. (Ifyou think the feature is badly thought out, feel free to send us reasons why you think it should bechanged, but don’t just send us mail saying ‘This is a bug’ if the documentation says we did it onpurpose.) Then read section 10.1, and don’t bother reporting the bug if it’s listed there.

If you do report a bug, please give us all of the following information:

• What operating system you’re running NASM under. DOS, Linux, NetBSD, Win16, Win32,VMS (I’d be impressed), whatever.

• If you’re running NASM under DOS or Win32, tell us whether you’ve compiled your ownexecutable from the DOS source archive, or whether you were using the standard distributionbinaries out of the archive. If you were using a locally built executable, try to reproduce theproblem using one of the standard binaries, as this will make it easier for us to reproduce yourproblem prior to fixing it.

• Which version of NASM you’re using, and exactly how you invoked it. Give us the precisecommand line, and the contents of the NASMENV environment variable if any.

• Which versions of any supplementary programs you’re using, and how you invoked them. If theproblem only becomes visible at link time, tell us what linker you’re using, what version of ityou’ve got, and the exact linker command line. If the problem involves linking against objectfiles generated by a compiler, tell us what compiler, what version, and what command line or

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options you used. (If you’re compiling in an IDE, please try to reproduce the problem with thecommand−line version of the compiler.)

• If at all possible, send us a NASM source file which exhibits the problem. If this causescopyright problems (e.g. you can only reproduce the bug in restricted−distribution code) thenbear in mind the following two points: firstly, we guarantee that any source code sent to us forthe purposes of debugging NASM will be used only for the purposes of debugging NASM, andthat we will delete all our copies of it as soon as we have found and fixed the bug or bugs inquestion; and secondly, we would prefer not to be mailed large chunks of code anyway. Thesmaller the file, the better. A three−line sample file that does nothing useful except demonstratethe problem is much easier to work with than a fully fledged ten−thousand−line program. (Ofcourse, some errors do only crop up in large files, so this may not be possible.)

• A description of what the problem actually is. ‘It doesn’t work’ is not a helpful description!Please describe exactly what is happening that shouldn’t be, or what isn’t happening that should.Examples might be: ‘NASM generates an error message saying Line 3 for an error that’s actuallyon Line 5’; ‘NASM generates an error message that I believe it shouldn’t be generating at all’;‘NASM fails to generate an error message that I believe it should be generating’; ‘the object fileproduced from this source code crashes my linker’; ‘the ninth byte of the output file is 66 and Ithink it should be 77 instead’.

• If you believe the output file from NASM to be faulty, send it to us. That allows us to determinewhether our own copy of NASM generates the same file, or whether the problem is related toportability issues between our development platforms and yours. We can handle binary filesmailed to us as MIME attachments, uuencoded, and even BinHex. Alternatively, we may be ableto provide an FTP site you can upload the suspect files to; but mailing them is easier for us.

• Any other information or data files that might be helpful. If, for example, the problem involvesNASM failing to generate an object file while TASM can generate an equivalent file withouttrouble, then send us both object files, so we can see what TASM is doing differently from us.

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Appendix A: Ndisasm

The Netwide Disassembler, NDISASM

A.1 IntroductionThe Netwide Disassembler is a small companion program to the Netwide Assembler, NASM. Itseemed a shame to have an x86 assembler, complete with a full instruction table, and not make asmuch use of it as possible, so here’s a disassembler which shares the instruction table (and someother bits of code) with NASM.

The Netwide Disassembler does nothing except to produce disassemblies of binary source files.NDISASM does not have any understanding of object file formats, like objdump , and it will notunderstand DOS .EXE files like debug will. It just disassembles.

A.2 Getting Started: InstallationSee section 1.3 for installation instructions. NDISASM, like NASM, has a man page which youmay want to put somewhere useful, if you are on a Unix system.

A.3 Running NDISASMTo disassemble a file, you will typically use a command of the form

ndisasm [−b16 | −b32] filename

NDISASM can disassemble 16−bit code or 32−bit code equally easily, provided of course that youremember to specify which it is to work with. If no −b switch is present, NDISASM works in16−bit mode by default. The −u switch (for USE32) also invokes 32−bit mode.

Two more command line options are −r which reports the version number of NDISASM you arerunning, and −h which gives a short summary of command line options.

A.3.1 COM Files: Specifying an OriginTo disassemble a DOS .COM file correctly, a disassembler must assume that the first instruction inthe file is loaded at address 0x100 , rather than at zero. NDISASM, which assumes by default thatany file you give it is loaded at zero, will therefore need to be informed of this.

The −o option allows you to declare a different origin for the file you are disassembling. Itsargument may be expressed in any of the NASM numeric formats: decimal by default, if it beginswith ‘$’ or ‘0x ’ or ends in ‘H’ it’s hex , if it ends in ‘Q’ it’s octal , and if it ends in ‘B’ it’sbinary .

Hence, to disassemble a .COM file:

ndisasm −o100h filename.com

will do the trick.

A.3.2 Code Following Data: SynchronisationSuppose you are disassembling a file which contains some data which isn’t machine code, and thencontains some machine code. NDISASM will faithfully plough through the data section, producingmachine instructions wherever it can (although most of them will look bizarre, and some may have

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unusual prefixes, e.g. ‘FS OR AX,0x240A ’), and generating ‘DB’ instructions ever so often ifit’s totally stumped. Then it will reach the code section.

Supposing NDISASM has just finished generating a strange machine instruction from part of thedata section, and its file position is now one byte before the beginning of the code section. It’sentirely possible that another spurious instruction will get generated, starting with the final byte ofthe data section, and then the correct first instruction in the code section will not be seen because thestarting point skipped over it. This isn’t really ideal.

To avoid this, you can specify a ‘synchronisation ’ point, or indeed as many synchronisationpoints as you like (although NDISASM can only handle 8192 sync points internally). The definitionof a sync point is this: NDISASM guarantees to hit sync points exactly during disassembly. If it isthinking about generating an instruction which would cause it to jump over a sync point, it willdiscard that instruction and output a ‘db ’ instead. So it will start disassembly exactly from the syncpoint, and so you will see all the instructions in your code section.

Sync points are specified using the −s option: they are measured in terms of the program origin, notthe file position. So if you want to synchronise after 32 bytes of a .COM file, you would have to do

ndisasm −o100h −s120h file.com

rather than

ndisasm −o100h −s20h file.com

As stated above, you can specify multiple sync markers if you need to, just by repeating the −soption.

A.3.3 Mixed Code and Data: Automatic (Intelligent) Synchronisation Suppose you are disassembling the boot sector of a DOS floppy (maybe it has a virus, and you needto understand the virus so that you know what kinds of damage it might have done you). Typically,this will contain a JMP instruction, then some data, then the rest of the code. So there is a very goodchance of NDISASM being misaligned when the data ends and the code begins. Hence a sync pointis needed.

On the other hand, why should you have to specify the sync point manually? What you’d do inorder to find where the sync point would be, surely, would be to read the JMP instruction, and thento use its target address as a sync point. So can NDISASM do that for you?

The answer, of course, is yes: using either of the synonymous switches −a (for automatic sync) or−i (for intelligent sync) will enable auto−sync mode. Auto−sync mode automatically generatesa sync point for any forward−referring PC−relative jump or call instruction that NDISASMencounters. (Since NDISASM is one−pass, if it encounters a PC−relative jump whose target hasalready been processed, there isn’t much it can do about it...)

Only PC−relative jumps are processed, since an absolute jump is either through a register (in whichcase NDISASM doesn’t know what the register contains) or involves a segment address (in whichcase the target code isn’t in the same segment that NDISASM is working in, and so the sync pointcan’t be placed anywhere useful).

For some kinds of file, this mechanism will automatically put sync points in all the right places, andsave you from having to place any sync points manually. However, it should be stressed thatauto−sync mode is not guaranteed to catch all the sync points, and you may still have to place somemanually.

Auto−sync mode doesn’t prevent you from declaring manual sync points: it just adds automaticallygenerated ones to the ones you provide. It’s perfectly feasible to specify −i and some −s options.

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Another caveat with auto−sync mode is that if, by some unpleasant fluke, something in your datasection should disassemble to a PC−relative call or jump instruction, NDISASM may obedientlyplace a sync point in a totally random place, for example in the middle of one of the instructions inyour code section. So you may end up with a wrong disassembly even if you use auto−sync. Again,there isn’t much I can do about this. If you have problems, you’ll have to use manual sync points, oruse the −k option (documented below) to suppress disassembly of the data area.

A.3.4 Other OptionsThe −e option skips a header on the file, by ignoring the first N bytes. This means that the header isnot counted towards the disassembly offset: if you give −e10 −o10 , disassembly will start at byte10 in the file, and this will be given offset 10, not 20.

The −k option is provided with two comma−separated numeric arguments, the first of which is anassembly offset and the second is a number of bytes to skip. This will count the skipped bytestowards the assembly offset: its use is to suppress disassembly of a data section which wouldn’tcontain anything you wanted to see anyway.

A.4 Bugs and ImprovementsThere are no known bugs. However, any you find, with patches if possible, should be sent [email protected] or [email protected] , or to the developer’s site athttps://sourceforge.net/projects/nasm/ and we’ll try to fix them. Feel free to sendcontributions and new features as well.

Future plans include awareness of which processors certain instructions will run on, and marking ofinstructions that are too advanced for some processor (or are FPU instructions, or are undocumentedopcodes, or are privileged protected−mode instructions, or whatever).

That’s All Folks!

I hope NDISASM is of some use to somebody. Including me. :−)

I don’t recommend taking NDISASM apart to see how an efficient disassembler works, because asfar as I know, it isn’t an efficient one anyway. You have been warned.

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Appendix B: x86 Instruction Reference

This appendix provides a complete list of the machine instructions which NASM will assemble, anda short description of the function of each one.

It is not intended to be exhaustive documentation on the fine details of the instructions’ function,such as which exceptions they can trigger: for such documentation, you should go to Intel’s Website, http://developer.intel.com/design/Pentium4/manuals/ .

Instead, this appendix is intended primarily to provide documentation on the way the instructionsmay be used within NASM. For example, looking up LOOP will tell you that NASM allows CX orECX to be specified as an optional second argument to the LOOP instruction, to enforce which of thetwo possible counter registers should be used if the default is not the one desired.

The instructions are not quite listed in alphabetical order, since groups of instructions with similarfunctions are lumped together in the same entry. Most of them don’t move very far from theiralphabetic position because of this.

B.1 Key to Operand SpecificationsThe instruction descriptions in this appendix specify their operands using the following notation:

• Registers: reg8 denotes an 8−bit general purpose register, reg16 denotes a 16−bit generalpurpose register, and reg32 a 32−bit one. fpureg denotes one of the eight FPU stack registers,mmxreg denotes one of the eight 64−bit MMX registers, and segreg denotes a segmentregister. In addition, some registers (such as AL, DX or ECX) may be specified explicitly.

• Immediate operands: imm denotes a generic immediate operand. imm8, imm16 and imm32 areused when the operand is intended to be a specific size. For some of these instructions, NASMneeds an explicit specifier: for example, ADD ESP,16 could be interpreted as eitherADD r/m32,imm32 or ADD r/m32,imm8 . NASM chooses the former by default, and so youmust specify ADD ESP,BYTE 16 for the latter.

• Memory references: mem denotes a generic memory reference; mem8, mem16, mem32, mem64and mem80 are used when the operand needs to be a specific size. Again, a specifier is needed insome cases: DEC [address] is ambiguous and will be rejected by NASM. You must specifyDEC BYTE [address] , DEC WORD [address] or DEC DWORD [address] instead.

• Restricted memory references: one form of the MOV instruction allows a memory address to bespecified without allowing the normal range of register combinations and effective addressprocessing. This is denoted by memoffs8 , memoffs16 and memoffs32 .

• Register or memory choices: many instructions can accept either a register or a memoryreference as an operand. r/m8 is a shorthand for reg8/mem8 ; similarly r/m16 and r/m32 .r/m64 is MMX−related, and is a shorthand for mmxreg/mem64.

B.2 Key to Opcode DescriptionsThis appendix also provides the opcodes which NASM will generate for each form of eachinstruction. The opcodes are listed in the following way:

• A hex number, such as 3F, indicates a fixed byte containing that number.

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• A hex number followed by +r , such as C8+r , indicates that one of the operands to theinstruction is a register, and the ‘register value’ of that register should be added to the hexnumber to produce the generated byte. For example, EDX has register value 2, so the codeC8+r , when the register operand is EDX, generates the hex byte CA. Register values for specificregisters are given in section B.2.1.

• A hex number followed by +cc , such as 40+cc , indicates that the instruction name has acondition code suffix, and the numeric representation of the condition code should be added tothe hex number to produce the generated byte. For example, the code 40+cc , when theinstruction contains the NE condition, generates the hex byte 45 . Condition codes and theirnumeric representations are given in section B.2.2.

• A slash followed by a digit, such as /2 , indicates that one of the operands to the instruction is amemory address or register (denoted mem or r/m , with an optional size). This is to be encoded asan effective address, with a ModR/M byte, an optional SIB byte, and an optional displacement,and the spare (register) field of the ModR/M byte should be the digit given (which will be from 0to 7, so it fits in three bits). The encoding of effective addresses is given in section B.2.5.

• The code /r combines the above two: it indicates that one of the operands is a memory addressor r/m , and another is a register, and that an effective address should be generated with the spare(register) field in the ModR/M byte being equal to the ‘register value’ of the register operand.The encoding of effective addresses is given in section B.2.5; register values are given in sectionB.2.1.

• The codes ib , iw and id indicate that one of the operands to the instruction is an immediatevalue, and that this is to be encoded as a byte, little−endian word or little−endian doublewordrespectively.

• The codes rb , rw and rd indicate that one of the operands to the instruction is an immediatevalue, and that the difference between this value and the address of the end of the instruction is tobe encoded as a byte, word or doubleword respectively. Where the form rw/rd appears, itindicates that either rw or rd should be used according to whether assembly is being performedin BITS 16 or BITS 32 state respectively.

• The codes ow and od indicate that one of the operands to the instruction is a reference to thecontents of a memory address specified as an immediate value: this encoding is used in someforms of the MOV instruction in place of the standard effective−address mechanism. Thedisplacement is encoded as a word or doubleword. Again, ow/od denotes that ow or od shouldbe chosen according to the BITS setting.

• The codes o16 and o32 indicate that the given form of the instruction should be assembled withoperand size 16 or 32 bits. In other words, o16 indicates a 66 prefix in BITS 32 state, butgenerates no code in BITS 16 state; and o32 indicates a 66 prefix in BITS 16 state butgenerates nothing in BITS 32 .

• The codes a16 and a32 , similarly to o16 and o32 , indicate the address size of the given formof the instruction. Where this does not match the BITS setting, a 67 prefix is required.

B.2.1 Register ValuesWhere an instruction requires a register value, it is already implicit in the encoding of the rest of theinstruction what type of register is intended: an 8−bit general−purpose register, a segment register, adebug register, an MMX register, or whatever. Therefore there is no problem with registers ofdifferent types sharing an encoding value.

The encodings for the various classes of register are:

• 8−bit general registers: AL is 0, CL is 1, DL is 2, BL is 3, AH is 4, CH is 5, DH is 6, and BH is 7.

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• 16−bit general registers: AX is 0, CX is 1, DX is 2, BX is 3, SP is 4, BP is 5, SI is 6, and DI is 7.

• 32−bit general registers: EAX is 0, ECX is 1, EDX is 2, EBX is 3, ESP is 4, EBP is 5, ESI is 6,and EDI is 7.

• Segment registers: ES is 0, CS is 1, SS is 2, DS is 3, FS is 4, and GS is 5.

• Floating−point registers: ST0 is 0, ST1 is 1, ST2 is 2, ST3 is 3, ST4 is 4, ST5 is 5, ST6 is 6,and ST7 is 7.

• 64−bit MMX registers: MM0 is 0, MM1 is 1, MM2 is 2, MM3 is 3, MM4 is 4, MM5 is 5, MM6 is 6, andMM7 is 7.

• Control registers: CR0 is 0, CR2 is 2, CR3 is 3, and CR4 is 4.

• Debug registers: DR0 is 0, DR1 is 1, DR2 is 2, DR3 is 3, DR6 is 6, and DR7 is 7.

• Test registers: TR3 is 3, TR4 is 4, TR5 is 5, TR6 is 6, and TR7 is 7.

(Note that wherever a register name contains a number, that number is also the register value forthat register.)

B.2.2 Condition CodesThe available condition codes are given here, along with their numeric representations as part ofopcodes. Many of these condition codes have synonyms, so several will be listed at a time.

In the following descriptions, the word ‘either’, when applied to two possible trigger conditions, isused to mean ‘either or both’. If ‘either but not both’ is meant, the phrase ‘exactly one of’ is used.

• O is 0 (trigger if the overflow flag is set); NO is 1.

• B, C and NAE are 2 (trigger if the carry flag is set); AE, NB and NC are 3.

• E and Z are 4 (trigger if the zero flag is set); NE and NZ are 5.

• BE and NA are 6 (trigger if either of the carry or zero flags is set); A and NBE are 7.

• S is 8 (trigger if the sign flag is set); NS is 9.

• P and PE are 10 (trigger if the parity flag is set); NP and PO are 11.

• L and NGE are 12 (trigger if exactly one of the sign and overflow flags is set); GE and NL are 13.

• LE and NG are 14 (trigger if either the zero flag is set, or exactly one of the sign and overflowflags is set); G and NLE are 15.

Note that in all cases, the sense of a condition code may be reversed by changing the low bit of thenumeric representation.

For details of when an instruction sets each of the status flags, see the individual instruction, plusthe Status Flags reference in section B.2.4

B.2.3 SSE Condition PredicatesThe condition predicates for SSE comparison instructions are the codes used as part of the opcode,to determine what form of comparison is being carried out. In each case, the imm8 value is the finalbyte of the opcode encoding, and the predicate is the code used as part of the mnemonic for theinstruction (equivalent to the "cc" in an integer instruction that used a condition code). Theinstructions that use this will give details of what the various mnemonics are, this table is used tohelp you work out details of what is happening.

Predi− imm8 Description Relation where: Emula− Result QNaN cate Encod− A Is 1st Operand tion if NaN Signal

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ing B Is 2nd Operand Operand Invalid

EQ 000B equal A = B False No

LT 001B less−than A < B False Yes

LE 010B less−than− A <= B False Yes or−equal

−−− −−−− greater A > B Swap False Yes than Operands, Use LT

−−− −−−− greater− A >= B Swap False Yes than−or−equal Operands, Use LE

UNORD 011B unordered A, B = Unordered True No

NEQ 100B not−equal A != B True No

NLT 101B not−less− NOT(A < B) True Yes than

NLE 110B not−less− NOT(A <= B) True Yes than−or− equal

−−− −−−− not−greater NOT(A > B) Swap True Yes than Operands, Use NLT

−−− −−−− not−greater NOT(A >= B) Swap True Yes than− Operands, or−equal Use NLE

ORD 111B ordered A , B = Ordered False No

The unordered relationship is true when at least one of the two values being compared is a NaN orin an unsupported format.

Note that the comparisons which are listed as not having a predicate or encoding can only beachieved through software emulation, as described in the "emulation" column. Note in particularthat an instruction such as greater−than is not the same as NLE, as, unlike with the CMPinstruction, it has to take into account the possibility of one operand containing a NaN or anunsupported numeric format.

B.2.4 Status FlagsThe status flags provide some information about the result of the arithmetic instructions. Thisinformation can be used by conditional instructions (such a Jcc and CMOVcc) as well as by someof the other instructions (such as ADC and INTO).

There are 6 status flags:

CF − Carry flag.

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Set if an arithmetic operation generates a carry or a borrow out of the most−significant bit of theresult; cleared otherwise. This flag indicates an overflow condition for unsigned−integer arithmetic.It is also used in multiple−precision arithmetic.

PF − Parity flag.

Set if the least−significant byte of the result contains an even number of 1 bits; cleared otherwise.

AF − Adjust flag.

Set if an arithmetic operation generates a carry or a borrow out of bit 3 of the result; clearedotherwise. This flag is used in binary−coded decimal (BCD) arithmetic.

ZF − Zero flag.

Set if the result is zero; cleared otherwise.

SF − Sign flag.

Set equal to the most−significant bit of the result, which is the sign bit of a signed integer. (0indicates a positive value and 1 indicates a negative value.)

OF − Overflow flag.

Set if the integer result is too large a positive number or too small a negative number (excluding thesign−bit) to fit in the destination operand; cleared otherwise. This flag indicates an overflowcondition for signed−integer (two’s complement) arithmetic.

B.2.5 Effective Address Encoding: ModR/M and SIBAn effective address is encoded in up to three parts: a ModR/M byte, an optional SIB byte, and anoptional byte, word or doubleword displacement field.

The ModR/M byte consists of three fields: the mod field, ranging from 0 to 3, in the upper two bitsof the byte, the r/m field, ranging from 0 to 7, in the lower three bits, and the spare (register) fieldin the middle (bit 3 to bit 5). The spare field is not relevant to the effective address being encoded,and either contains an extension to the instruction opcode or the register value of another operand.

The ModR/M system can be used to encode a direct register reference rather than a memory access.This is always done by setting the mod field to 3 and the r/m field to the register value of theregister in question (it must be a general−purpose register, and the size of the register must alreadybe implicit in the encoding of the rest of the instruction). In this case, the SIB byte and displacementfield are both absent.

In 16−bit addressing mode (either BITS 16 with no 67 prefix, or BITS 32 with a 67 prefix), theSIB byte is never used. The general rules for mod and r/m (there is an exception, given below) are:

• The mod field gives the length of the displacement field: 0 means no displacement, 1 means onebyte, and 2 means two bytes.

• The r/m field encodes the combination of registers to be added to the displacement to give theaccessed address: 0 means BX+SI , 1 means BX+DI, 2 means BP+SI , 3 means BP+DI, 4 meansSI only, 5 means DI only, 6 means BP only, and 7 means BX only.

However, there is a special case:

• If mod is 0 and r/m is 6, the effective address encoded is not [BP] as the above rules wouldsuggest, but instead [disp16] : the displacement field is present and is two bytes long, and noregisters are added to the displacement.

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Therefore the effective address [BP] cannot be encoded as efficiently as [BX] ; so if you code[BP] in a program, NASM adds a notional 8−bit zero displacement, and sets mod to 1, r/m to 6,and the one−byte displacement field to 0.

In 32−bit addressing mode (either BITS 16 with a 67 prefix, or BITS 32 with no 67 prefix) thegeneral rules (again, there are exceptions) for mod and r/m are:

• The mod field gives the length of the displacement field: 0 means no displacement, 1 means onebyte, and 2 means four bytes.

• If only one register is to be added to the displacement, and it is not ESP, the r/m field gives itsregister value, and the SIB byte is absent. If the r/m field is 4 (which would encode ESP), theSIB byte is present and gives the combination and scaling of registers to be added to thedisplacement.

If the SIB byte is present, it describes the combination of registers (an optional base register, and anoptional index register scaled by multiplication by 1, 2, 4 or 8) to be added to the displacement. TheSIB byte is divided into the scale field, in the top two bits, the index field in the next three, andthe base field in the bottom three. The general rules are:

• The base field encodes the register value of the base register.

• The index field encodes the register value of the index register, unless it is 4, in which case noindex register is used (so ESP cannot be used as an index register).

• The scale field encodes the multiplier by which the index register is scaled before adding it tothe base and displacement: 0 encodes a multiplier of 1, 1 encodes 2, 2 encodes 4 and 3 encodes 8.

The exceptions to the 32−bit encoding rules are:

• If mod is 0 and r/m is 5, the effective address encoded is not [EBP] as the above rules wouldsuggest, but instead [disp32] : the displacement field is present and is four bytes long, and noregisters are added to the displacement.

• If mod is 0, r/m is 4 (meaning the SIB byte is present) and base is 4, the effective addressencoded is not [EBP+index] as the above rules would suggest, but instead[disp32+index] : the displacement field is present and is four bytes long, and there is nobase register (but the index register is still processed in the normal way).

B.3 Key to Instruction FlagsGiven along with each instruction in this appendix is a set of flags, denoting the type of theinstruction. The types are as follows:

• 8086 , 186 , 286 , 386 , 486 , PENT and P6 denote the lowest processor type that supports theinstruction. Most instructions run on all processors above the given type; those that do not aredocumented. The Pentium II contains no additional instructions beyond the P6 (Pentium Pro);from the point of view of its instruction set, it can be thought of as a P6 with MMX capability.

• 3DNOW indicates that the instruction is a 3DNow! one, and will run on the AMD K6−2 and laterprocessors. ATHLON extensions to the 3DNow! instruction set are documented as such.

• CYRIX indicates that the instruction is specific to Cyrix processors, for example the extra MMXinstructions in the Cyrix extended MMX instruction set.

• FPU indicates that the instruction is a floating−point one, and will only run on machines with acoprocessor (automatically including 486DX, Pentium and above).

• KATMAI indicates that the instruction was introduced as part of the Katmai New Instruction set.These instructions are available on the Pentium III and later processors. Those which are notspecifically SSE instructions are also available on the AMD Athlon.

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• MMX indicates that the instruction is an MMX one, and will run on MMX−capable Pentiumprocessors and the Pentium II.

• PRIV indicates that the instruction is a protected−mode management instruction. Many of thesemay only be used in protected mode, or only at privilege level zero.

• SSE and SSE2 indicate that the instruction is a Streaming SIMD Extension instruction. Theseinstructions operate on multiple values in a single operation. SSE was introduced with thePentium III and SSE2 was introduced with the Pentium 4.

• UNDOC indicates that the instruction is an undocumented one, and not part of the official IntelArchitecture; it may or may not be supported on any given machine.

• WILLAMETTE indicates that the instruction was introduced as part of the new instruction set inthe Pentium 4 and Intel Xeon processors. These instructions are also known as SSE2 instructions.

B.4 x86 Instruction SetB.4.1 AAA, AAS, AAM, AAD: ASCII Adjustments

AAA ; 37 [8086]

AAS ; 3F [8086]

AAD ; D5 0A [8086] AAD imm ; D5 ib [8086]

AAM ; D4 0A [8086] AAM imm ; D4 ib [8086]

These instructions are used in conjunction with the add, subtract, multiply and divide instructions toperform binary−coded decimal arithmetic in unpacked (one BCD digit per byte – easy to translate toand from ASCII , hence the instruction names) form. There are also packed BCD instructions DAAand DAS: see section B.4.57.

• AAA (ASCII Adjust After Addition) should be used after a one−byte ADD instruction whosedestination was the AL register: by means of examining the value in the low nibble of AL andalso the auxiliary carry flag AF, it determines whether the addition has overflowed, and adjusts it(and sets the carry flag) if so. You can add long BCD strings together by doing ADD/AAA on thelow digits, then doing ADC/AAA on each subsequent digit.

• AAS (ASCII Adjust AL After Subtraction) works similarly to AAA, but is for use after SUBinstructions rather than ADD.

• AAM (ASCII Adjust AX After Multiply) is for use after you have multiplied two decimal digitstogether and left the result in AL: it divides AL by ten and stores the quotient in AH, leaving theremainder in AL. The divisor 10 can be changed by specifying an operand to the instruction: aparticularly handy use of this is AAM 16, causing the two nibbles in AL to be separated into AHand AL.

• AAD (ASCII Adjust AX Before Division) performs the inverse operation to AAM: it multiplies AHby ten, adds it to AL, and sets AH to zero. Again, the multiplier 10 can be changed.

B.4.2 ADC: Add with CarryADC r/m8,reg8 ; 10 /r [8086] ADC r/m16,reg16 ; o16 11 /r [8086] ADC r/m32,reg32 ; o32 11 /r [386]

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ADC reg8,r/m8 ; 12 /r [8086] ADC reg16,r/m16 ; o16 13 /r [8086] ADC reg32,r/m32 ; o32 13 /r [386]

ADC r/m8,imm8 ; 80 /2 ib [8086] ADC r/m16,imm16 ; o16 81 /2 iw [8086] ADC r/m32,imm32 ; o32 81 /2 id [386]

ADC r/m16,imm8 ; o16 83 /2 ib [8086] ADC r/m32,imm8 ; o32 83 /2 ib [386]

ADC AL,imm8 ; 14 ib [8086] ADC AX,imm16 ; o16 15 iw [8086] ADC EAX,imm32 ; o32 15 id [386]

ADC performs integer addition: it adds its two operands together, plus the value of the carry flag,and leaves the result in its destination (first) operand. The destination operand can be a register or amemory location. The source operand can be a register, a memory location or an immediate value.

The flags are set according to the result of the operation: in particular, the carry flag is affected andcan be used by a subsequent ADC instruction.

In the forms with an 8−bit immediate second operand and a longer first operand, the second operandis considered to be signed, and is sign−extended to the length of the first operand. In these cases, theBYTE qualifier is necessary to force NASM to generate this form of the instruction.

To add two numbers without also adding the contents of the carry flag, use ADD (section B.4.3).

B.4.3 ADD: Add IntegersADD r/m8,reg8 ; 00 /r [8086] ADD r/m16,reg16 ; o16 01 /r [8086] ADD r/m32,reg32 ; o32 01 /r [386]

ADD reg8,r/m8 ; 02 /r [8086] ADD reg16,r/m16 ; o16 03 /r [8086] ADD reg32,r/m32 ; o32 03 /r [386]

ADD r/m8,imm8 ; 80 /0 ib [8086] ADD r/m16,imm16 ; o16 81 /0 iw [8086] ADD r/m32,imm32 ; o32 81 /0 id [386]

ADD r/m16,imm8 ; o16 83 /0 ib [8086] ADD r/m32,imm8 ; o32 83 /0 ib [386]

ADD AL,imm8 ; 04 ib [8086] ADD AX,imm16 ; o16 05 iw [8086] ADD EAX,imm32 ; o32 05 id [386]

ADD performs integer addition: it adds its two operands together, and leaves the result in itsdestination (first) operand. The destination operand can be a register or a memory location. Thesource operand can be a register, a memory location or an immediate value.

The flags are set according to the result of the operation: in particular, the carry flag is affected andcan be used by a subsequent ADC instruction.


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B.4.4 ADDPD: ADD Packed Double−Precision FP ValuesADDPD xmm1,xmm2/mem128 ; 66 0F 58 /r [WILLAMETTE,SSE2]

ADDPD performs addition on each of two packed double−precision FP value pairs.

dst[0−63] := dst[0−63] + src[0−63], dst[64−127] := dst[64−127] + src[64−127].

The destination is an XMM register. The source operand can be either an XMM register or a 128−bitmemory location.

B.4.5 ADDPS: ADD Packed Single−Precision FP ValuesADDPS xmm1,xmm2/mem128 ; 0F 58 /r [KATMAI,SSE]

ADDPS performs addition on each of four packed single−precision FP value pairs

dst[0−31] := dst[0−31] + src[0−31], dst[32−63] := dst[32−63] + src[32−63], dst[64−95] := dst[64−95] + src[64−95], dst[96−127] := dst[96−127] + src[96−127].


B.4.6 ADDSD: ADD Scalar Double−Precision FP ValuesADDSD xmm1,xmm2/mem64 ; F2 0F 58 /r [KATMAI,SSE]

ADDSD adds the low double−precision FP values from the source and destination operands andstores the double−precision FP result in the destination operand.

dst[0−63] := dst[0−63] + src[0−63], dst[64−127) remains unchanged.


B.4.7 ADDSS: ADD Scalar Single−Precision FP ValuesADDSS xmm1,xmm2/mem32 ; F3 0F 58 /r [WILLAMETTE,SSE2]

ADDSS adds the low single−precision FP values from the source and destination operands andstores the single−precision FP result in the destination operand.

dst[0−31] := dst[0−31] + src[0−31], dst[32−127] remains unchanged.


B.4.8 AND: Bitwise ANDAND r/m8,reg8 ; 20 /r [8086] AND r/m16,reg16 ; o16 21 /r [8086] AND r/m32,reg32 ; o32 21 /r [386]

AND reg8,r/m8 ; 22 /r [8086] AND reg16,r/m16 ; o16 23 /r [8086] AND reg32,r/m32 ; o32 23 /r [386]

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AND r/m8,imm8 ; 80 /4 ib [8086] AND r/m16,imm16 ; o16 81 /4 iw [8086] AND r/m32,imm32 ; o32 81 /4 id [386]

AND r/m16,imm8 ; o16 83 /4 ib [8086] AND r/m32,imm8 ; o32 83 /4 ib [386]

AND AL,imm8 ; 24 ib [8086] AND AX,imm16 ; o16 25 iw [8086] AND EAX,imm32 ; o32 25 id [386]

AND performs a bitwise AND operation between its two operands (i.e. each bit of the result is 1 ifand only if the corresponding bits of the two inputs were both 1), and stores the result in thedestination (first) operand. The destination operand can be a register or a memory location. Thesource operand can be a register, a memory location or an immediate value.


The MMX instruction PAND (see section B.4.202) performs the same operation on the 64−bit MMXregisters.

B.4.9 ANDNPD: Bitwise Logical AND NOT of Packed Double−Precision FP ValuesANDNPD xmm1,xmm2/mem128 ; 66 0F 55 /r [WILLAMETTE,SSE2]

ANDNPD inverts the bits of the two double−precision floating−point values in the destinationregister, and then performs a logical AND between the two double−precision floating−point valuesin the source operand and the temporary inverted result, storing the result in the destination register.

dst[0−63] := src[0−63] AND NOT dst[0−63], dst[64−127] := src[64−127] AND NOT dst[64−127].


B.4.10 ANDNPS: Bitwise Logical AND NOT of Packed Single−Precision FP ValuesANDNPS xmm1,xmm2/mem128 ; 0F 55 /r [KATMAI,SSE]

ANDNPS inverts the bits of the four single−precision floating−point values in the destinationregister, and then performs a logical AND between the four single−precision floating−point valuesin the source operand and the temporary inverted result, storing the result in the destination register.

dst[0−31] := src[0−31] AND NOT dst[0−31], dst[32−63] := src[32−63] AND NOT dst[32−63], dst[64−95] := src[64−95] AND NOT dst[64−95], dst[96−127] := src[96−127] AND NOT dst[96−127].


B.4.11 ANDPD: Bitwise Logical AND For Single FPANDPD xmm1,xmm2/mem128 ; 66 0F 54 /r [WILLAMETTE,SSE2]

ANDPD performs a bitwise logical AND of the two double−precision floating point values in thesource and destination operand, and stores the result in the destination register.

dst[0−63] := src[0−63] AND dst[0−63], dst[64−127] := src[64−127] AND dst[64−127].

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B.4.12 ANDPS: Bitwise Logical AND For Single FPANDPS xmm1,xmm2/mem128 ; 0F 54 /r [KATMAI,SSE]

ANDPS performs a bitwise logical AND of the four single−precision floating point values in thesource and destination operand, and stores the result in the destination register.

dst[0−31] := src[0−31] AND dst[0−31], dst[32−63] := src[32−63] AND dst[32−63], dst[64−95] := src[64−95] AND dst[64−95], dst[96−127] := src[96−127] AND dst[96−127].


B.4.13 ARPL: Adjust RPL Field of SelectorARPL r/m16,reg16 ; 63 /r [286,PRIV]

ARPL expects its two word operands to be segment selectors. It adjusts the RPL (requested privilegelevel – stored in the bottom two bits of the selector) field of the destination (first) operand to ensurethat it is no less (i.e. no more privileged than) the RPL field of the source operand. The zero flag isset if and only if a change had to be made.

B.4.14 BOUND: Check Array Index against BoundsBOUND reg16,mem ; o16 62 /r [186] BOUND reg32,mem ; o32 62 /r [386]

BOUND expects its second operand to point to an area of memory containing two signed values ofthe same size as its first operand (i.e. two words for the 16−bit form; two doublewords for the32−bit form). It performs two signed comparisons: if the value in the register passed as its firstoperand is less than the first of the in−memory values, or is greater than or equal to the second, itthrows a BR exception. Otherwise, it does nothing.

B.4.15 BSF, BSR: Bit ScanBSF reg16,r/m16 ; o16 0F BC /r [386] BSF reg32,r/m32 ; o32 0F BC /r [386]

BSR reg16,r/m16 ; o16 0F BD /r [386] BSR reg32,r/m32 ; o32 0F BD /r [386]

• BSF searches for the least significant set bit in its source (second) operand, and if it finds one,stores the index in its destination (first) operand. If no set bit is found, the contents of thedestination operand are undefined. If the source operand is zero, the zero flag is set.

• BSR performs the same function, but searches from the top instead, so it finds the mostsignificant set bit.

Bit indices are from 0 (least significant) to 15 or 31 (most significant). The destination operand canonly be a register. The source operand can be a register or a memory location.

B.4.16 BSWAP: Byte SwapBSWAP reg32 ; o32 0F C8+r [486]

BSWAP swaps the order of the four bytes of a 32−bit register: bits 0−7 exchange places with bits24−31, and bits 8−15 swap with bits 16−23. There is no explicit 16−bit equivalent: to byte−swap

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AX, BX, CX or DX, XCHG can be used. When BSWAP is used with a 16−bit register, the result isundefined.

B.4.17 BT, BTC, BTR, BTS: Bit TestBT r/m16,reg16 ; o16 0F A3 /r [386] BT r/m32,reg32 ; o32 0F A3 /r [386] BT r/m16,imm8 ; o16 0F BA /4 ib [386] BT r/m32,imm8 ; o32 0F BA /4 ib [386]

BTC r/m16,reg16 ; o16 0F BB /r [386] BTC r/m32,reg32 ; o32 0F BB /r [386] BTC r/m16,imm8 ; o16 0F BA /7 ib [386] BTC r/m32,imm8 ; o32 0F BA /7 ib [386]

BTR r/m16,reg16 ; o16 0F B3 /r [386] BTR r/m32,reg32 ; o32 0F B3 /r [386] BTR r/m16,imm8 ; o16 0F BA /6 ib [386] BTR r/m32,imm8 ; o32 0F BA /6 ib [386]

BTS r/m16,reg16 ; o16 0F AB /r [386] BTS r/m32,reg32 ; o32 0F AB /r [386] BTS r/m16,imm ; o16 0F BA /5 ib [386] BTS r/m32,imm ; o32 0F BA /5 ib [386]

These instructions all test one bit of their first operand, whose index is given by the second operand,and store the value of that bit into the carry flag. Bit indices are from 0 (least significant) to 15 or 31(most significant).

In addition to storing the original value of the bit into the carry flag, BTR also resets (clears) the bitin the operand itself. BTS sets the bit, and BTC complements the bit. BT does not modify itsoperands.

The destination can be a register or a memory location. The source can be a register or animmediate value.

If the destination operand is a register, the bit offset should be in the range 0−15 (for 16−bitoperands) or 0−31 (for 32−bit operands). An immediate value outside these ranges will be takenmodulo 16/32 by the processor.

If the destination operand is a memory location, then an immediate bit offset follows the same rulesas for a register. If the bit offset is in a register, then it can be anything within the signed range ofthe register used (ie, for a 32−bit operand, it can be (−2^31) to (2^31 – 1)

B.4.18 CALL: Call SubroutineCALL imm ; E8 rw/rd [8086] CALL imm:imm16 ; o16 9A iw iw [8086] CALL imm:imm32 ; o32 9A id iw [386] CALL FAR mem16 ; o16 FF /3 [8086] CALL FAR mem32 ; o32 FF /3 [386] CALL r/m16 ; o16 FF /2 [8086] CALL r/m32 ; o32 FF /2 [386]

CALL calls a subroutine, by means of pushing the current instruction pointer (IP ) and optionally CSas well on the stack, and then jumping to a given address.

CS is pushed as well as IP if and only if the call is a far call, i.e. a destination segment address isspecified in the instruction. The forms involving two colon−separated arguments are far calls; so arethe CALL FAR mem forms.

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The immediate near call takes one of two forms (call imm16/imm32 , determined by the currentsegment size limit. For 16−bit operands, you would use CALL 0x1234 , and for 32−bit operandsyou would use CALL 0x12345678 . The value passed as an operand is a relative offset.

You can choose between the two immediate far call forms (CALL imm:imm ) by the use of theWORD and DWORD keywords: CALL WORD 0x1234:0x5678 ) orCALL DWORD 0x1234:0x56789abc .

The CALL FAR mem forms execute a far call by loading the destination address out of memory.The address loaded consists of 16 or 32 bits of offset (depending on the operand size), and 16 bits ofsegment. The operand size may be overridden using CALL WORD FAR mem orCALL DWORD FAR mem.

The CALL r/m forms execute a near call (within the same segment), loading the destinationaddress out of memory or out of a register. The keyword NEAR may be specified, for clarity, inthese forms, but is not necessary. Again, operand size can be overridden using CALL WORD memor CALL DWORD mem.

As a convenience, NASM does not require you to call a far procedure symbol by coding thecumbersome CALL SEG routine:routine , but instead allows the easier synonymCALL FAR routine .

The CALL r/m forms given above are near calls; NASM will accept the NEAR keyword (e.g.CALL NEAR [address] ), even though it is not strictly necessary.

B.4.19 CBW, CWD, CDQ, CWDE: Sign ExtensionsCBW ; o16 98 [8086] CWDE ; o32 98 [386]

CWD ; o16 99 [8086] CDQ ; o32 99 [386]

All these instructions sign−extend a short value into a longer one, by replicating the top bit of theoriginal value to fill the extended one.

CBW extends AL into AX by repeating the top bit of AL in every bit of AH. CWDE extends AX intoEAX. CWD extends AX into DX:AX by repeating the top bit of AX throughout DX, and CDQ extendsEAX into EDX:EAX.

B.4.20 CLC, CLD, CLI , CLTS: Clear FlagsCLC ; F8 [8086] CLD ; FC [8086] CLI ; FA [8086] CLTS ; 0F 06 [286,PRIV]

These instructions clear various flags. CLC clears the carry flag; CLD clears the direction flag; CLIclears the interrupt flag (thus disabling interrupts); and CLTS clears the task−switched (TS) flag inCR0.

To set the carry, direction, or interrupt flags, use the STC, STD and STI instructions (sectionB.4.301). To invert the carry flag, use CMC (section B.4.22).

B.4.21 CLFLUSH: Flush Cache LineCLFLUSH mem ; 0F AE /7 [WILLAMETTE,SSE2]

CLFLUSH invalidates the cache line that contains the linear address specified by the source operandfrom all levels of the processor cache hierarchy (data and instruction). If, at any level of the cache

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hierarchy, the line is inconsistent with memory (dirty) it is written to memory before invalidation.The source operand points to a byte−sized memory location.

Although CLFLUSH is flagged SSE2 and above, it may not be present on all processors which haveSSE2 support, and it may be supported on other processors; the CPUID instruction (section B.4.34)will return a bit which indicates support for the CLFLUSH instruction.

B.4.22 CMC: Complement Carry FlagCMC ; F5 [8086]

CMC changes the value of the carry flag: if it was 0, it sets it to 1, and vice versa.

B.4.23 CMOVcc: Conditional MoveCMOVcc reg16,r/m16 ; o16 0F 40+cc /r [P6] CMOVcc reg32,r/m32 ; o32 0F 40+cc /r [P6]

CMOV moves its source (second) operand into its destination (first) operand if the given conditioncode is satisfied; otherwise it does nothing.

For a list of condition codes, see section B.2.2.

Although the CMOV instructions are flagged P6 and above, they may not be supported by allPentium Pro processors; the CPUID instruction (section B.4.34) will return a bit which indicateswhether conditional moves are supported.

B.4.24 CMP: Compare IntegersCMP r/m8,reg8 ; 38 /r [8086] CMP r/m16,reg16 ; o16 39 /r [8086] CMP r/m32,reg32 ; o32 39 /r [386]

CMP reg8,r/m8 ; 3A /r [8086] CMP reg16,r/m16 ; o16 3B /r [8086] CMP reg32,r/m32 ; o32 3B /r [386]

CMP r/m8,imm8 ; 80 /0 ib [8086] CMP r/m16,imm16 ; o16 81 /0 iw [8086] CMP r/m32,imm32 ; o32 81 /0 id [386]

CMP r/m16,imm8 ; o16 83 /0 ib [8086] CMP r/m32,imm8 ; o32 83 /0 ib [386]

CMP AL,imm8 ; 3C ib [8086] CMP AX,imm16 ; o16 3D iw [8086] CMP EAX,imm32 ; o32 3D id [386]

CMP performs a ‘mental’ subtraction of its second operand from its first operand, and affects theflags as if the subtraction had taken place, but does not store the result of the subtraction anywhere.


The destination operand can be a register or a memory location. The source can be a register,memory location or an immediate value of the same size as the destination.

B.4.25 CMPccPD: Packed Double−Precision FP Compare CMPPD xmm1,xmm2/mem128,imm8 ; 66 0F C2 /r ib [WILLAMETTE,SSE2]

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CMPEQPD xmm1,xmm2/mem128 ; 66 0F C2 /r 00 [WILLAMETTE,SSE2] CMPLTPD xmm1,xmm2/mem128 ; 66 0F C2 /r 01 [WILLAMETTE,SSE2] CMPLEPD xmm1,xmm2/mem128 ; 66 0F C2 /r 02 [WILLAMETTE,SSE2] CMPUNORDPD xmm1,xmm2/mem128 ; 66 0F C2 /r 03 [WILLAMETTE,SSE2] CMPNEQPD xmm1,xmm2/mem128 ; 66 0F C2 /r 04 [WILLAMETTE,SSE2] CMPNLTPD xmm1,xmm2/mem128 ; 66 0F C2 /r 05 [WILLAMETTE,SSE2] CMPNLEPD xmm1,xmm2/mem128 ; 66 0F C2 /r 06 [WILLAMETTE,SSE2] CMPORDPD xmm1,xmm2/mem128 ; 66 0F C2 /r 07 [WILLAMETTE,SSE2]

The CMPccPD instructions compare the two packed double−precision FP values in the source anddestination operands, and returns the result of the comparison in the destination register. The resultof each comparison is a quadword mask of all 1s (comparison true) or all 0s (comparison false).

The destination is an XMM register. The source can be either an XMM register or a 128−bit memorylocation.

The third operand is an 8−bit immediate value, of which the low 3 bits define the type ofcomparison. For ease of programming, the 8 two−operand pseudo−instructions are provided, withthe third operand already filled in. The Condition Predicates are:

EQ 0 Equal LT 1 Less−than LE 2 Less−than−or−equal UNORD 3 Unordered NE 4 Not−equal NLT 5 Not−less−than NLE 6 Not−less−than−or−equal ORD 7 Ordered

For more details of the comparison predicates, and details of how to emulate the "greater−than"equivalents, see section B.2.3

B.4.26 CMPccPS: Packed Single−Precision FP Compare CMPPS xmm1,xmm2/mem128,imm8 ; 0F C2 /r ib [KATMAI,SSE]

CMPEQPS xmm1,xmm2/mem128 ; 0F C2 /r 00 [KATMAI,SSE] CMPLTPS xmm1,xmm2/mem128 ; 0F C2 /r 01 [KATMAI,SSE] CMPLEPS xmm1,xmm2/mem128 ; 0F C2 /r 02 [KATMAI,SSE] CMPUNORDPS xmm1,xmm2/mem128 ; 0F C2 /r 03 [KATMAI,SSE] CMPNEQPS xmm1,xmm2/mem128 ; 0F C2 /r 04 [KATMAI,SSE] CMPNLTPS xmm1,xmm2/mem128 ; 0F C2 /r 05 [KATMAI,SSE] CMPNLEPS xmm1,xmm2/mem128 ; 0F C2 /r 06 [KATMAI,SSE] CMPORDPS xmm1,xmm2/mem128 ; 0F C2 /r 07 [KATMAI,SSE]

The CMPccPS instructions compare the two packed single−precision FP values in the source anddestination operands, and returns the result of the comparison in the destination register. The resultof each comparison is a doubleword mask of all 1s (comparison true) or all 0s (comparison false).



EQ 0 Equal LT 1 Less−than LE 2 Less−than−or−equal

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UNORD 3 Unordered NE 4 Not−equal NLT 5 Not−less−than NLE 6 Not−less−than−or−equal ORD 7 Ordered


B.4.27 CMPSB, CMPSW, CMPSD: Compare StringsCMPSB ; A6 [8086] CMPSW ; o16 A7 [8086] CMPSD ; o32 A7 [386]

CMPSB compares the byte at [DS:SI] or [DS:ESI] with the byte at [ES:DI] or [ES:EDI] ,and sets the flags accordingly. It then increments or decrements (depending on the direction flag:increments if the flag is clear, decrements if it is set) SI and DI (or ESI and EDI ).

The registers used are SI and DI if the address size is 16 bits, and ESI and EDI if it is 32 bits. Ifyou need to use an address size not equal to the current BITS setting, you can use an explicit a16or a32 prefix.

The segment register used to load from [SI] or [ESI] can be overridden by using a segmentregister name as a prefix (for example, ES CMPSB). The use of ES for the load from [DI] or[EDI] cannot be overridden.

CMPSW and CMPSD work in the same way, but they compare a word or a doubleword instead of abyte, and increment or decrement the addressing registers by 2 or 4 instead of 1.

The REPE and REPNE prefixes (equivalently, REPZ and REPNZ) may be used to repeat theinstruction up to CX (or ECX – again, the address size chooses which) times until the first unequal orequal byte is found.

B.4.28 CMPccSD: Scalar Double−Precision FP Compare CMPSD xmm1,xmm2/mem64,imm8 ; F2 0F C2 /r ib [WILLAMETTE,SSE2]

CMPEQSD xmm1,xmm2/mem64 ; F2 0F C2 /r 00 [WILLAMETTE,SSE2] CMPLTSD xmm1,xmm2/mem64 ; F2 0F C2 /r 01 [WILLAMETTE,SSE2] CMPLESD xmm1,xmm2/mem64 ; F2 0F C2 /r 02 [WILLAMETTE,SSE2] CMPUNORDSD xmm1,xmm2/mem64 ; F2 0F C2 /r 03 [WILLAMETTE,SSE2] CMPNEQSD xmm1,xmm2/mem64 ; F2 0F C2 /r 04 [WILLAMETTE,SSE2] CMPNLTSD xmm1,xmm2/mem64 ; F2 0F C2 /r 05 [WILLAMETTE,SSE2] CMPNLESD xmm1,xmm2/mem64 ; F2 0F C2 /r 06 [WILLAMETTE,SSE2] CMPORDSD xmm1,xmm2/mem64 ; F2 0F C2 /r 07 [WILLAMETTE,SSE2]

The CMPccSD instructions compare the low−order double−precision FP values in the source anddestination operands, and returns the result of the comparison in the destination register. The resultof each comparison is a quadword mask of all 1s (comparison true) or all 0s (comparison false).



EQ 0 Equal LT 1 Less−than

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LE 2 Less−than−or−equal UNORD 3 Unordered NE 4 Not−equal NLT 5 Not−less−than NLE 6 Not−less−than−or−equal ORD 7 Ordered


B.4.29 CMPccSS: Scalar Single−Precision FP Compare CMPSS xmm1,xmm2/mem32,imm8 ; F3 0F C2 /r ib [KATMAI,SSE]

CMPEQSS xmm1,xmm2/mem32 ; F3 0F C2 /r 00 [KATMAI,SSE] CMPLTSS xmm1,xmm2/mem32 ; F3 0F C2 /r 01 [KATMAI,SSE] CMPLESS xmm1,xmm2/mem32 ; F3 0F C2 /r 02 [KATMAI,SSE] CMPUNORDSS xmm1,xmm2/mem32 ; F3 0F C2 /r 03 [KATMAI,SSE] CMPNEQSS xmm1,xmm2/mem32 ; F3 0F C2 /r 04 [KATMAI,SSE] CMPNLTSS xmm1,xmm2/mem32 ; F3 0F C2 /r 05 [KATMAI,SSE] CMPNLESS xmm1,xmm2/mem32 ; F3 0F C2 /r 06 [KATMAI,SSE] CMPORDSS xmm1,xmm2/mem32 ; F3 0F C2 /r 07 [KATMAI,SSE]

The CMPccSS instructions compare the low−order single−precision FP values in the source anddestination operands, and returns the result of the comparison in the destination register. The resultof each comparison is a doubleword mask of all 1s (comparison true) or all 0s (comparison false).



EQ 0 Equal LT 1 Less−than LE 2 Less−than−or−equal UNORD 3 Unordered NE 4 Not−equal NLT 5 Not−less−than NLE 6 Not−less−than−or−equal ORD 7 Ordered


B.4.30 CMPXCHG, CMPXCHG486: Compare and ExchangeCMPXCHG r/m8,reg8 ; 0F B0 /r [PENT] CMPXCHG r/m16,reg16 ; o16 0F B1 /r [PENT] CMPXCHG r/m32,reg32 ; o32 0F B1 /r [PENT]

CMPXCHG486 r/m8,reg8 ; 0F A6 /r [486,UNDOC] CMPXCHG486 r/m16,reg16 ; o16 0F A7 /r [486,UNDOC] CMPXCHG486 r/m32,reg32 ; o32 0F A7 /r [486,UNDOC]

These two instructions perform exactly the same operation; however, apparently some (not all) 486processors support it under a non−standard opcode, so NASM provides the undocumentedCMPXCHG486 form to generate the non−standard opcode.

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CMPXCHG compares its destination (first) operand to the value in AL, AX or EAX (depending on theoperand size of the instruction). If they are equal, it copies its source (second) operand into thedestination and sets the zero flag. Otherwise, it clears the zero flag and copies the destinationregister to AL, AX or EAX.

The destination can be either a register or a memory location. The source is a register.

CMPXCHG is intended to be used for atomic operations in multitasking or multiprocessorenvironments. To safely update a value in shared memory, for example, you might load the valueinto EAX, load the updated value into EBX, and then execute the instructionLOCK CMPXCHG [value],EBX . If value has not changed since being loaded, it is updatedwith your desired new value, and the zero flag is set to let you know it has worked. (The LOCKprefix prevents another processor doing anything in the middle of this operation: it guaranteesatomicity.) However, if another processor has modified the value in between your load and yourattempted store, the store does not happen, and you are notified of the failure by a cleared zero flag,so you can go round and try again.

B.4.31 CMPXCHG8B: Compare and Exchange Eight BytesCMPXCHG8B mem ; 0F C7 /1 [PENT]

This is a larger and more unwieldy version of CMPXCHG: it compares the 64−bit (eight−byte) valuestored at [mem] with the value in EDX:EAX. If they are equal, it sets the zero flag and storesECX:EBX into the memory area. If they are unequal, it clears the zero flag and stores the memorycontents into EDX:EAX.

CMPXCHG8B can be used with the LOCK prefix, to allow atomic execution. This is useful inmulti−processor and multi−tasking environments.

B.4.32 COMISD: Scalar Ordered Double−Precision FP Compare and Set EFLAGSCOMISD xmm1,xmm2/mem64 ; 66 0F 2F /r [WILLAMETTE,SSE2]

COMISD compares the low−order double−precision FP value in the two source operands. ZF, PFand CF are set according to the result. OF, AF and AF are cleared. The unordered result is returnedif either source is a NaN (QNaN or SNaN).

The destination operand is an XMM register. The source can be either an XMM register or a memorylocation.

The flags are set according to the following rules:

Result Flags Values

UNORDERED: ZF,PF,CF <−− 111; GREATER_THAN: ZF,PF,CF <−− 000; LESS_THAN: ZF,PF,CF <−− 001; EQUAL: ZF,PF,CF <−− 100;

B.4.33 COMISS: Scalar Ordered Single−Precision FP Compare and Set EFLAGSCOMISS xmm1,xmm2/mem32 ; 66 0F 2F /r [KATMAI,SSE]

COMISS compares the low−order single−precision FP value in the two source operands. ZF, PF andCF are set according to the result. OF, AF and AF are cleared. The unordered result is returned ifeither source is a NaN (QNaN or SNaN).

The destination operand is an XMM register. The source can be either an XMM register or a memorylocation.

The flags are set according to the following rules:

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Result Flags Values

UNORDERED: ZF,PF,CF <−− 111; GREATER_THAN: ZF,PF,CF <−− 000; LESS_THAN: ZF,PF,CF <−− 001; EQUAL: ZF,PF,CF <−− 100;

B.4.34 CPUID: Get CPU Identification CodeCPUID ; 0F A2 [PENT]

CPUID returns various information about the processor it is being executed on. It fills the fourregisters EAX, EBX, ECX and EDX with information, which varies depending on the input contentsof EAX.

CPUID also acts as a barrier to serialise instruction execution: executing the CPUID instructionguarantees that all the effects (memory modification, flag modification, register modification) ofprevious instructions have been completed before the next instruction gets fetched.

The information returned is as follows:

• If EAX is zero on input, EAX on output holds the maximum acceptable input value of EAX, andEBX:EDX:ECX contain the string "GenuineIntel" (or not, if you have a clone processor).That is to say, EBX contains "Genu" (in NASM’s own sense of character constants, described insection 3.4.2), EDX contains "ineI" and ECX contains "ntel" .

• If EAX is one on input, EAX on output contains version information about the processor, and EDXcontains a set of feature flags, showing the presence and absence of various features. Forexample, bit 8 is set if the CMPXCHG8B instruction (section B.4.31) is supported, bit 15 is set ifthe conditional move instructions (section B.4.23 and section B.4.72) are supported, and bit 23 isset if MMX instructions are supported.

• If EAX is two on input, EAX, EBX, ECX and EDX all contain information about caches and TLBs(Translation Lookahead Buffers).

For more information on the data returned from CPUID, see the documentation from Intel and otherprocessor manufacturers.

B.4.35 CVTDQ2PD: Packed Signed INT32 to Packed Double−Precision FP ConversionCVTDQ2PD xmm1,xmm2/mem64 ; F3 0F E6 /r [WILLAMETTE,SSE2]

CVTDQ2PD converts two packed signed doublewords from the source operand to two packeddouble−precision FP values in the destination operand.

The destination operand is an XMM register. The source can be either an XMM register or a 64−bitmemory location. If the source is a register, the packed integers are in the low quadword.

B.4.36 CVTDQ2PS: Packed Signed INT32 to Packed Single−Precision FP ConversionCVTDQ2PS xmm1,xmm2/mem128 ; 0F 5B /r [WILLAMETTE,SSE2]

CVTDQ2PS converts four packed signed doublewords from the source operand to four packedsingle−precision FP values in the destination operand.

The destination operand is an XMM register. The source can be either an XMM register or a 128−bitmemory location.

For more details of this instruction, see the Intel Processor manuals.

B.4.37 CVTPD2DQ: Packed Double−Precision FP to Packed Signed INT32 ConversionCVTPD2DQ xmm1,xmm2/mem128 ; F2 0F E6 /r [WILLAMETTE,SSE2]

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CVTPD2DQ converts two packed double−precision FP values from the source operand to twopacked signed doublewords in the low quadword of the destination operand. The high quadword ofthe destination is set to all 0s.



B.4.38 CVTPD2PI: Packed Double−Precision FP to Packed Signed INT32 ConversionCVTPD2PI mm,xmm/mem128 ; 66 0F 2D /r [WILLAMETTE,SSE2]

CVTPD2PI converts two packed double−precision FP values from the source operand to twopacked signed doublewords in the destination operand.

The destination operand is an MMX register. The source can be either an XMM register or a 128−bitmemory location.


B.4.39 CVTPD2PS: Packed Double−Precision FP to Packed Single−Precision FP ConversionCVTPD2PS xmm1,xmm2/mem128 ; 66 0F 5A /r [WILLAMETTE,SSE2]

CVTPD2PS converts two packed double−precision FP values from the source operand to twopacked single−precision FP values in the low quadword of the destination operand. The highquadword of the destination is set to all 0s.



B.4.40 CVTPI2PD: Packed Signed INT32 to Packed Double−Precision FP ConversionCVTPI2PD xmm,mm/mem64 ; 66 0F 2A /r [WILLAMETTE,SSE2]

CVTPI2PD converts two packed signed doublewords from the source operand to two packeddouble−precision FP values in the destination operand.

The destination operand is an XMM register. The source can be either an MMX register or a 64−bitmemory location.


B.4.41 CVTPI2PS: Packed Signed INT32 to Packed Single−FP ConversionCVTPI2PS xmm,mm/mem64 ; 0F 2A /r [KATMAI,SSE]

CVTPI2PS converts two packed signed doublewords from the source operand to two packedsingle−precision FP values in the low quadword of the destination operand. The high quadword ofthe destination remains unchanged.

The destination operand is an XMM register. The source can be either an MMX register or a 64−bitmemory location.


B.4.42 CVTPS2DQ: Packed Single−Precision FP to Packed Signed INT32 ConversionCVTPS2DQ xmm1,xmm2/mem128 ; 66 0F 5B /r [WILLAMETTE,SSE2]

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CVTPS2DQ converts four packed single−precision FP values from the source operand to fourpacked signed doublewords in the destination operand.



B.4.43 CVTPS2PD: Packed Single−Precision FP to Packed Double−Precision FP ConversionCVTPS2PD xmm1,xmm2/mem64 ; 0F 5A /r [WILLAMETTE,SSE2]

CVTPS2PD converts two packed single−precision FP values from the source operand to two packeddouble−precision FP values in the destination operand.

The destination operand is an XMM register. The source can be either an XMM register or a 64−bitmemory location. If the source is a register, the input values are in the low quadword.


B.4.44 CVTPS2PI: Packed Single−Precision FP to Packed Signed INT32 ConversionCVTPS2PI mm,xmm/mem64 ; 0F 2D /r [KATMAI,SSE]

CVTPS2PI converts two packed single−precision FP values from the source operand to two packedsigned doublewords in the destination operand.

The destination operand is an MMX register. The source can be either an XMM register or a 64−bitmemory location. If the source is a register, the input values are in the low quadword.


B.4.45 CVTSD2SI: Scalar Double−Precision FP to Signed INT32 ConversionCVTSD2SI reg32,xmm/mem64 ; F2 0F 2D /r [WILLAMETTE,SSE2]

CVTSD2SI converts a double−precision FP value from the source operand to a signed doublewordin the destination operand.

The destination operand is a general purpose register. The source can be either an XMM register or a64−bit memory location. If the source is a register, the input value is in the low quadword.


B.4.46 CVTSD2SS: Scalar Double−Precision FP to Scalar Single−Precision FP ConversionCVTSD2SS xmm1,xmm2/mem64 ; F2 0F 5A /r [KATMAI,SSE]

CVTSD2SS converts a double−precision FP value from the source operand to a single−precision FPvalue in the low doubleword of the destination operand. The upper 3 doublewords are leftunchanged.

The destination operand is an XMM register. The source can be either an XMM register or a 64−bitmemory location. If the source is a register, the input value is in the low quadword.


B.4.47 CVTSI2SD: Signed INT32 to Scalar Double−Precision FP ConversionCVTSI2SD xmm,r/m32 ; F2 0F 2A /r [WILLAMETTE,SSE2]

CVTSI2SD converts a signed doubleword from the source operand to a double−precision FP valuein the low quadword of the destination operand. The high quadword is left unchanged.

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The destination operand is an XMM register. The source can be either a general purpose register or a32−bit memory location.


B.4.48 CVTSI2SS: Signed INT32 to Scalar Single−Precision FP ConversionCVTSI2SS xmm,r/m32 ; F3 0F 2A /r [KATMAI,SSE]

CVTSI2SS converts a signed doubleword from the source operand to a single−precision FP valuein the low doubleword of the destination operand. The upper 3 doublewords are left unchanged.

The destination operand is an XMM register. The source can be either a general purpose register or a32−bit memory location.


B.4.49 CVTSS2SD: Scalar Single−Precision FP to Scalar Double−Precision FP ConversionCVTSS2SD xmm1,xmm2/mem32 ; F3 0F 5A /r [WILLAMETTE,SSE2]

CVTSS2SD converts a single−precision FP value from the source operand to a double−precision FPvalue in the low quadword of the destination operand. The upper quadword is left unchanged.

The destination operand is an XMM register. The source can be either an XMM register or a 32−bitmemory location. If the source is a register, the input value is contained in the low doubleword.


B.4.50 CVTSS2SI: Scalar Single−Precision FP to Signed INT32 ConversionCVTSS2SI reg32,xmm/mem32 ; F3 0F 2D /r [KATMAI,SSE]

CVTSS2SI converts a single−precision FP value from the source operand to a signed doublewordin the destination operand.

The destination operand is a general purpose register. The source can be either an XMM register or a32−bit memory location. If the source is a register, the input value is in the low doubleword.


B.4.51 CVTTPD2DQ: Packed Double−Precision FP to Packed Signed INT32 Conversion withTruncationCVTTPD2DQ xmm1,xmm2/mem128 ; 66 0F E6 /r [WILLAMETTE,SSE2]

CVTTPD2DQ converts two packed double−precision FP values in the source operand to two packedsingle−precision FP values in the destination operand. If the result is inexact, it is truncated(rounded toward zero). The high quadword is set to all 0s.



B.4.52 CVTTPD2PI: Packed Double−Precision FP to Packed Signed INT32 Conversion withTruncationCVTTPD2PI mm,xmm/mem128 ; 66 0F 2C /r [WILLAMETTE,SSE2]

CVTTPD2PI converts two packed double−precision FP values in the source operand to two packedsingle−precision FP values in the destination operand. If the result is inexact, it is truncated(rounded toward zero).

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The destination operand is an MMX register. The source can be either an XMM register or a 128−bitmemory location.


B.4.53 CVTTPS2DQ: Packed Single−Precision FP to Packed Signed INT32 Conversion withTruncationCVTTPS2DQ xmm1,xmm2/mem128 ; F3 0F 5B /r [WILLAMETTE,SSE2]

CVTTPS2DQ converts four packed single−precision FP values in the source operand to four packedsigned doublewords in the destination operand. If the result is inexact, it is truncated (roundedtoward zero).



B.4.54 CVTTPS2PI: Packed Single−Precision FP to Packed Signed INT32 Conversion withTruncationCVTTPS2PI mm,xmm/mem64 ; 0F 2C /r [KATMAI,SSE]

CVTTPS2PI converts two packed single−precision FP values in the source operand to two packedsigned doublewords in the destination operand. If the result is inexact, it is truncated (roundedtoward zero). If the source is a register, the input values are in the low quadword.

The destination operand is an MMX register. The source can be either an XMM register or a 64−bitmemory location. If the source is a register, the input value is in the low quadword.


B.4.55 CVTTSD2SI: Scalar Double−Precision FP to Signed INT32 Conversion withTruncationCVTTSD2SI reg32,xmm/mem64 ; F2 0F 2C /r [WILLAMETTE,SSE2]

CVTTSD2SI converts a double−precision FP value in the source operand to a signed doubleword inthe destination operand. If the result is inexact, it is truncated (rounded toward zero).

The destination operand is a general purpose register. The source can be either an XMM register or a64−bit memory location. If the source is a register, the input value is in the low quadword.


B.4.56 CVTTSS2SI: Scalar Single−Precision FP to Signed INT32 Conversion withTruncationCVTTSD2SI reg32,xmm/mem32 ; F3 0F 2C /r [KATMAI,SSE]

CVTTSS2SI converts a single−precision FP value in the source operand to a signed doubleword inthe destination operand. If the result is inexact, it is truncated (rounded toward zero).

The destination operand is a general purpose register. The source can be either an XMM register or a32−bit memory location. If the source is a register, the input value is in the low doubleword.


B.4.57 DAA, DAS: Decimal AdjustmentsDAA ; 27 [8086] DAS ; 2F [8086]

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These instructions are used in conjunction with the add and subtract instructions to performbinary−coded decimal arithmetic in packed (one BCD digit per nibble) form. For the unpackedequivalents, see section B.4.1.

DAA should be used after a one−byte ADD instruction whose destination was the AL register: bymeans of examining the value in the AL and also the auxiliary carry flag AF, it determines whethereither digit of the addition has overflowed, and adjusts it (and sets the carry and auxiliary−carryflags) if so. You can add long BCD strings together by doing ADD/DAA on the low two digits, thendoing ADC/DAA on each subsequent pair of digits.

DAS works similarly to DAA, but is for use after SUB instructions rather than ADD.

B.4.58 DEC: Decrement IntegerDEC reg16 ; o16 48+r [8086] DEC reg32 ; o32 48+r [386] DEC r/m8 ; FE /1 [8086] DEC r/m16 ; o16 FF /1 [8086] DEC r/m32 ; o32 FF /1 [386]

DEC subtracts 1 from its operand. It does not affect the carry flag: to affect the carry flag, useSUB something,1 (see section B.4.305). DEC affects all the other flags according to the result.

This instruction can be used with a LOCK prefix to allow atomic execution.

See also INC (section B.4.120).

B.4.59 DIV : Unsigned Integer DivideDIV r/m8 ; F6 /6 [8086] DIV r/m16 ; o16 F7 /6 [8086] DIV r/m32 ; o32 F7 /6 [386]

DIV performs unsigned integer division. The explicit operand provided is the divisor; the dividendand destination operands are implicit, in the following way:

• For DIV r/m8 , AX is divided by the given operand; the quotient is stored in AL and theremainder in AH.

• For DIV r/m16 , DX:AX is divided by the given operand; the quotient is stored in AX and theremainder in DX.

• For DIV r/m32 , EDX:EAX is divided by the given operand; the quotient is stored in EAX andthe remainder in EDX.

Signed integer division is performed by the IDIV instruction: see section B.4.117.

B.4.60 DIVPD: Packed Double−Precision FP DivideDIVPD xmm1,xmm2/mem128 ; 66 0F 5E /r [WILLAMETTE,SSE2]

DIVPD divides the two packed double−precision FP values in the destination operand by the twopacked double−precision FP values in the source operand, and stores the packed double−precisionresults in the destination register.


dst[0−63] := dst[0−63] / src[0−63], dst[64−127] := dst[64−127] / src[64−127].

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B.4.61 DIVPS: Packed Single−Precision FP DivideDIVPS xmm1,xmm2/mem128 ; 0F 5E /r [KATMAI,SSE]

DIVPS divides the four packed single−precision FP values in the destination operand by the fourpacked single−precision FP values in the source operand, and stores the packed single−precisionresults in the destination register.


dst[0−31] := dst[0−31] / src[0−31], dst[32−63] := dst[32−63] / src[32−63], dst[64−95] := dst[64−95] / src[64−95], dst[96−127] := dst[96−127] / src[96−127].

B.4.62 DIVSD: Scalar Double−Precision FP DivideDIVSD xmm1,xmm2/mem64 ; F2 0F 5E /r [WILLAMETTE,SSE2]

DIVSD divides the low−order double−precision FP value in the destination operand by thelow−order double−precision FP value in the source operand, and stores the double−precision resultin the destination register.


dst[0−63] := dst[0−63] / src[0−63], dst[64−127] remains unchanged.

B.4.63 DIVSS: Scalar Single−Precision FP DivideDIVSS xmm1,xmm2/mem32 ; F3 0F 5E /r [KATMAI,SSE]

DIVSS divides the low−order single−precision FP value in the destination operand by thelow−order single−precision FP value in the source operand, and stores the single−precision result inthe destination register.


dst[0−31] := dst[0−31] / src[0−31], dst[32−127] remains unchanged.

B.4.64 EMMS: Empty MMX StateEMMS ; 0F 77 [PENT,MMX]

EMMS sets the FPU tag word (marking which floating−point registers are available) to all ones,meaning all registers are available for the FPU to use. It should be used after executing MMXinstructions and before executing any subsequent floating−point operations.

B.4.65 ENTER: Create Stack FrameENTER imm,imm ; C8 iw ib [186]

ENTER constructs a stack frame for a high−level language procedure call. The first operand(the iw in the opcode definition above refers to the first operand) gives the amount of stack space toallocate for local variables; the second (the ib above) gives the nesting level of the procedure (forlanguages like Pascal, with nested procedures).

The function of ENTER, with a nesting level of zero, is equivalent to

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PUSH EBP ; or PUSH BP in 16 bits MOV EBP,ESP ; or MOV BP,SP in 16 bits SUB ESP,operand1 ; or SUB SP,operand1 in 16 bits

This creates a stack frame with the procedure parameters accessible upwards from EBP, and localvariables accessible downwards from EBP.

With a nesting level of one, the stack frame created is 4 (or 2) bytes bigger, and the value of thefinal frame pointer EBP is accessible in memory at [EBP−4] .

This allows ENTER, when called with a nesting level of two, to look at the stack frame described bythe previous value of EBP, find the frame pointer at offset –4 from that, and push it along with itsnew frame pointer, so that when a level−two procedure is called from within a level−one procedure,[EBP−4] holds the frame pointer of the most recent level−one procedure call and [EBP−8] holdsthat of the most recent level−two call. And so on, for nesting levels up to 31.

Stack frames created by ENTER can be destroyed by the LEAVE instruction: see section B.4.136.

B.4.66 F2XM1: Calculate 2**X−1F2XM1 ; D9 F0 [8086,FPU]

F2XM1 raises 2 to the power of ST0, subtracts one, and stores the result back into ST0. The initialcontents of ST0 must be a number in the range –1.0 to +1.0.

B.4.67 FABS: Floating−Point Absolute ValueFABS ; D9 E1 [8086,FPU]

FABS computes the absolute value of ST0,by clearing the sign bit, and stores the result back in ST0.

B.4.68 FADD, FADDP: Floating−Point AdditionFADD mem32 ; D8 /0 [8086,FPU] FADD mem64 ; DC /0 [8086,FPU]

FADD fpureg ; D8 C0+r [8086,FPU] FADD ST0,fpureg ; D8 C0+r [8086,FPU]

FADD TO fpureg ; DC C0+r [8086,FPU] FADD fpureg,ST0 ; DC C0+r [8086,FPU]

FADDP fpureg ; DE C0+r [8086,FPU] FADDP fpureg,ST0 ; DE C0+r [8086,FPU]

• FADD, given one operand, adds the operand to ST0 and stores the result back in ST0. If theoperand has the TO modifier, the result is stored in the register given rather than in ST0.

• FADDP performs the same function as FADD TO, but pops the register stack after storing theresult.

The given two−operand forms are synonyms for the one−operand forms.

To add an integer value to ST0, use the c{FIADD} instruction (section B.4.80)

B.4.69 FBLD, FBSTP: BCD Floating−Point Load and StoreFBLD mem80 ; DF /4 [8086,FPU] FBSTP mem80 ; DF /6 [8086,FPU]

FBLD loads an 80−bit (ten−byte) packed binary−coded decimal number from the given memoryaddress, converts it to a real, and pushes it on the register stack. FBSTP stores the value of ST0, inpacked BCD, at the given address and then pops the register stack.

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B.4.70 FCHS: Floating−Point Change SignFCHS ; D9 E0 [8086,FPU]

FCHS negates the number in ST0, by inverting the sign bit: negative numbers become positive, andvice versa.

B.4.71 FCLEX, FNCLEX: Clear Floating−Point ExceptionsFCLEX ; 9B DB E2 [8086,FPU] FNCLEX ; DB E2 [8086,FPU]

FCLEX clears any floating−point exceptions which may be pending. FNCLEX does the same thingbut doesn’t wait for previous floating−point operations (including the handling of pendingexceptions) to finish first.

B.4.72 FCMOVcc: Floating−Point Conditional MoveFCMOVB fpureg ; DA C0+r [P6,FPU] FCMOVB ST0,fpureg ; DA C0+r [P6,FPU]

FCMOVE fpureg ; DA C8+r [P6,FPU] FCMOVE ST0,fpureg ; DA C8+r [P6,FPU]

FCMOVBE fpureg ; DA D0+r [P6,FPU] FCMOVBE ST0,fpureg ; DA D0+r [P6,FPU]

FCMOVU fpureg ; DA D8+r [P6,FPU] FCMOVU ST0,fpureg ; DA D8+r [P6,FPU]

FCMOVNB fpureg ; DB C0+r [P6,FPU] FCMOVNB ST0,fpureg ; DB C0+r [P6,FPU]

FCMOVNE fpureg ; DB C8+r [P6,FPU] FCMOVNE ST0,fpureg ; DB C8+r [P6,FPU]

FCMOVNBE fpureg ; DB D0+r [P6,FPU] FCMOVNBE ST0,fpureg ; DB D0+r [P6,FPU]

FCMOVNU fpureg ; DB D8+r [P6,FPU] FCMOVNU ST0,fpureg ; DB D8+r [P6,FPU]

The FCMOV instructions perform conditional move operations: each of them moves the contents ofthe given register into ST0 if its condition is satisfied, and does nothing if not.

The conditions are not the same as the standard condition codes used with conditional jumpinstructions. The conditions B, BE, NB, NBE, E and NE are exactly as normal, but none of the otherstandard ones are supported. Instead, the condition U and its counterpart NU are provided; the Ucondition is satisfied if the last two floating−point numbers compared were unordered, i.e. theywere not equal but neither one could be said to be greater than the other, for example if they wereNaNs. (The flag state which signals this is the setting of the parity flag: so the U condition isnotionally equivalent to PE, and NU is equivalent to PO.)

The FCMOV conditions test the main processor’s status flags, not the FPU status flags, so usingFCMOV directly after FCOM will not work. Instead, you should either use FCOMI which writesdirectly to the main CPU flags word, or use FSTSW to extract the FPU flags.

Although the FCMOV instructions are flagged P6 above, they may not be supported by all PentiumPro processors; the CPUID instruction (section B.4.34) will return a bit which indicates whetherconditional moves are supported.

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B.4.73 FCOM, FCOMP, FCOMPP, FCOMI, FCOMIP: Floating−Point CompareFCOM mem32 ; D8 /2 [8086,FPU] FCOM mem64 ; DC /2 [8086,FPU] FCOM fpureg ; D8 D0+r [8086,FPU] FCOM ST0,fpureg ; D8 D0+r [8086,FPU]

FCOMP mem32 ; D8 /3 [8086,FPU] FCOMP mem64 ; DC /3 [8086,FPU] FCOMP fpureg ; D8 D8+r [8086,FPU] FCOMP ST0,fpureg ; D8 D8+r [8086,FPU]

FCOMPP ; DE D9 [8086,FPU]

FCOMI fpureg ; DB F0+r [P6,FPU] FCOMI ST0,fpureg ; DB F0+r [P6,FPU]

FCOMIP fpureg ; DF F0+r [P6,FPU] FCOMIP ST0,fpureg ; DF F0+r [P6,FPU]

FCOM compares ST0 with the given operand, and sets the FPU flags accordingly. ST0 is treated asthe left−hand side of the comparison, so that the carry flag is set (for a ‘less−than’ result) if ST0 isless than the given operand.

FCOMP does the same as FCOM, but pops the register stack afterwards. FCOMPP compares ST0with ST1 and then pops the register stack twice.

FCOMI and FCOMIP work like the corresponding forms of FCOM and FCOMP, but write theirresults directly to the CPU flags register rather than the FPU status word, so they can beimmediately followed by conditional jump or conditional move instructions.

The FCOM instructions differ from the FUCOM instructions (section B.4.108) only in the way theyhandle quiet NaNs: FUCOM will handle them silently and set the condition code flags to an‘unordered’ result, whereas FCOM will generate an exception.

B.4.74 FCOS: CosineFCOS ; D9 FF [386,FPU]

FCOS computes the cosine of ST0 (in radians), and stores the result in ST0. The absolute value ofST0 must be less than 2**63.

See also FSINCOS (section B.4.100).

B.4.75 FDECSTP: Decrement Floating−Point Stack PointerFDECSTP ; D9 F6 [8086,FPU]

FDECSTP decrements the ‘top’ field in the floating−point status word. This has the effect ofrotating the FPU register stack by one, as if the contents of ST7 had been pushed on the stack. Seealso FINCSTP (section B.4.85).

B.4.76 FxDISI , FxENI : Disable and Enable Floating−Point InterruptsFDISI ; 9B DB E1 [8086,FPU] FNDISI ; DB E1 [8086,FPU]

FENI ; 9B DB E0 [8086,FPU] FNENI ; DB E0 [8086,FPU]

FDISI and FENI disable and enable floating−point interrupts. These instructions are onlymeaningful on original 8087 processors: the 287 and above treat them as no−operation instructions.

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FNDISI and FNENI do the same thing as FDISI and FENI respectively, but without waiting forthe floating−point processor to finish what it was doing first.

B.4.77 FDIV , FDIVP, FDIVR, FDIVRP: Floating−Point DivisionFDIV mem32 ; D8 /6 [8086,FPU] FDIV mem64 ; DC /6 [8086,FPU]

FDIV fpureg ; D8 F0+r [8086,FPU] FDIV ST0,fpureg ; D8 F0+r [8086,FPU]

FDIV TO fpureg ; DC F8+r [8086,FPU] FDIV fpureg,ST0 ; DC F8+r [8086,FPU]

FDIVR mem32 ; D8 /0 [8086,FPU] FDIVR mem64 ; DC /0 [8086,FPU]

FDIVR fpureg ; D8 F8+r [8086,FPU] FDIVR ST0,fpureg ; D8 F8+r [8086,FPU]

FDIVR TO fpureg ; DC F0+r [8086,FPU] FDIVR fpureg,ST0 ; DC F0+r [8086,FPU]

FDIVP fpureg ; DE F8+r [8086,FPU] FDIVP fpureg,ST0 ; DE F8+r [8086,FPU]

FDIVRP fpureg ; DE F0+r [8086,FPU] FDIVRP fpureg,ST0 ; DE F0+r [8086,FPU]

• FDIV divides ST0 by the given operand and stores the result back in ST0, unless the TOqualifier is given, in which case it divides the given operand by ST0 and stores the result in theoperand.

• FDIVR does the same thing, but does the division the other way up: so if TO is not given, itdivides the given operand by ST0 and stores the result in ST0, whereas if TO is given it dividesST0 by its operand and stores the result in the operand.

• FDIVP operates like FDIV TO , but pops the register stack once it has finished.

• FDIVRP operates like FDIVR TO , but pops the register stack once it has finished.

For FP/Integer divisions, see FIDIV (section B.4.82).

B.4.78 FEMMS: Faster Enter/Exit of the MMX or floating−point stateFEMMS ; 0F 0E [PENT,3DNOW]

FEMMS can be used in place of the EMMS instruction on processors which support the 3DNow!instruction set. Following execution of FEMMS, the state of the MMX/FP registers is undefined, andthis allows a faster context switch between FP and MMX instructions. The FEMMS instruction canalso be used before executing MMX instructions

B.4.79 FFREE: Flag Floating−Point Register as UnusedFFREE fpureg ; DD C0+r [8086,FPU] FFREEP fpureg ; DF C0+r [286,FPU,UNDOC]

FFREE marks the given register as being empty.

FFREEP marks the given register as being empty, and then pops the register stack.

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B.4.80 FIADD: Floating−Point/Integer AdditionFIADD mem16 ; DE /0 [8086,FPU] FIADD mem32 ; DA /0 [8086,FPU]

FIADD adds the 16−bit or 32−bit integer stored in the given memory location to ST0, storing theresult in ST0.

B.4.81 FICOM, FICOMP: Floating−Point/Integer CompareFICOM mem16 ; DE /2 [8086,FPU] FICOM mem32 ; DA /2 [8086,FPU]

FICOMP mem16 ; DE /3 [8086,FPU] FICOMP mem32 ; DA /3 [8086,FPU]

FICOM compares ST0 with the 16−bit or 32−bit integer stored in the given memory location, andsets the FPU flags accordingly. FICOMP does the same, but pops the register stack afterwards.

B.4.82 FIDIV , FIDIVR : Floating−Point/Integer DivisionFIDIV mem16 ; DE /6 [8086,FPU] FIDIV mem32 ; DA /6 [8086,FPU]

FIDIVR mem16 ; DE /7 [8086,FPU] FIDIVR mem32 ; DA /7 [8086,FPU]

FIDIV divides ST0 by the 16−bit or 32−bit integer stored in the given memory location, and storesthe result in ST0. FIDIVR does the division the other way up: it divides the integer by ST0, butstill stores the result in ST0.

B.4.83 FILD , FIST , FISTP : Floating−Point/Integer ConversionFILD mem16 ; DF /0 [8086,FPU] FILD mem32 ; DB /0 [8086,FPU] FILD mem64 ; DF /5 [8086,FPU]

FIST mem16 ; DF /2 [8086,FPU] FIST mem32 ; DB /2 [8086,FPU]

FISTP mem16 ; DF /3 [8086,FPU] FISTP mem32 ; DB /3 [8086,FPU] FISTP mem64 ; DF /7 [8086,FPU]

FILD loads an integer out of a memory location, converts it to a real, and pushes it on the FPUregister stack. FIST converts ST0 to an integer and stores that in memory; FISTP does the sameas FIST , but pops the register stack afterwards.

B.4.84 FIMUL: Floating−Point/Integer MultiplicationFIMUL mem16 ; DE /1 [8086,FPU] FIMUL mem32 ; DA /1 [8086,FPU]

FIMUL multiplies ST0 by the 16−bit or 32−bit integer stored in the given memory location, andstores the result in ST0.

B.4.85 FINCSTP: Increment Floating−Point Stack PointerFINCSTP ; D9 F7 [8086,FPU]

FINCSTP increments the ‘top’ field in the floating−point status word. This has the effect of rotatingthe FPU register stack by one, as if the register stack had been popped; however, unlike the popping

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of the stack performed by many FPU instructions, it does not flag the new ST7 (previously ST0) asempty. See also FDECSTP (section B.4.75).

B.4.86 FINIT , FNINIT : Initialise Floating−Point UnitFINIT ; 9B DB E3 [8086,FPU] FNINIT ; DB E3 [8086,FPU]

FINIT initialises the FPU to its default state. It flags all registers as empty, without actually changetheir values, clears the top of stack pointer. FNINIT does the same, without first waiting forpending exceptions to clear.

B.4.87 FISUB: Floating−Point/Integer SubtractionFISUB mem16 ; DE /4 [8086,FPU] FISUB mem32 ; DA /4 [8086,FPU]

FISUBR mem16 ; DE /5 [8086,FPU] FISUBR mem32 ; DA /5 [8086,FPU]

FISUB subtracts the 16−bit or 32−bit integer stored in the given memory location from ST0, andstores the result in ST0. FISUBR does the subtraction the other way round, i.e. it subtracts ST0from the given integer, but still stores the result in ST0.

B.4.88 FLD: Floating−Point LoadFLD mem32 ; D9 /0 [8086,FPU] FLD mem64 ; DD /0 [8086,FPU] FLD mem80 ; DB /5 [8086,FPU] FLD fpureg ; D9 C0+r [8086,FPU]

FLD loads a floating−point value out of the given register or memory location, and pushes it on theFPU register stack.

B.4.89 FLDxx : Floating−Point Load ConstantsFLD1 ; D9 E8 [8086,FPU] FLDL2E ; D9 EA [8086,FPU] FLDL2T ; D9 E9 [8086,FPU] FLDLG2 ; D9 EC [8086,FPU] FLDLN2 ; D9 ED [8086,FPU] FLDPI ; D9 EB [8086,FPU] FLDZ ; D9 EE [8086,FPU]

These instructions push specific standard constants on the FPU register stack.

Instruction Constant pushed

FLD1 1 FLDL2E base−2 logarithm of e FLDL2T base−2 log of 10 FLDLG2 base−10 log of 2 FLDLN2 base−e log of 2 FLDPI pi FLDZ zero

B.4.90 FLDCW: Load Floating−Point Control WordFLDCW mem16 ; D9 /5 [8086,FPU]

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FLDCW loads a 16−bit value out of memory and stores it into the FPU control word (governingthings like the rounding mode, the precision, and the exception masks). See also FSTCW (sectionB.4.103). If exceptions are enabled and you don’t want to generate one, use FCLEX or FNCLEX(section B.4.71) before loading the new control word.

B.4.91 FLDENV: Load Floating−Point EnvironmentFLDENV mem ; D9 /4 [8086,FPU]

FLDENV loads the FPU operating environment (control word, status word, tag word, instructionpointer, data pointer and last opcode) from memory. The memory area is 14 or 28 bytes long,depending on the CPU mode at the time. See also FSTENV (section B.4.104).

B.4.92 FMUL, FMULP: Floating−Point MultiplyFMUL mem32 ; D8 /1 [8086,FPU] FMUL mem64 ; DC /1 [8086,FPU]

FMUL fpureg ; D8 C8+r [8086,FPU] FMUL ST0,fpureg ; D8 C8+r [8086,FPU]

FMUL TO fpureg ; DC C8+r [8086,FPU] FMUL fpureg,ST0 ; DC C8+r [8086,FPU]

FMULP fpureg ; DE C8+r [8086,FPU] FMULP fpureg,ST0 ; DE C8+r [8086,FPU]

FMUL multiplies ST0 by the given operand, and stores the result in ST0, unless the TO qualifier isused in which case it stores the result in the operand. FMULP performs the same operation asFMUL TO, and then pops the register stack.

B.4.93 FNOP: Floating−Point No OperationFNOP ; D9 D0 [8086,FPU]

FNOP does nothing.

B.4.94 FPATAN, FPTAN: Arctangent and TangentFPATAN ; D9 F3 [8086,FPU] FPTAN ; D9 F2 [8086,FPU]

FPATAN computes the arctangent, in radians, of the result of dividing ST1 by ST0, stores the resultin ST1, and pops the register stack. It works like the C atan2 function, in that changing the sign ofboth ST0 and ST1 changes the output value by pi (so it performs true rectangular−to−polarcoordinate conversion, with ST1 being the Y coordinate and ST0 being the X coordinate, notmerely an arctangent).

FPTAN computes the tangent of the value in ST0 (in radians), and stores the result back into ST0.

The absolute value of ST0 must be less than 2**63.

B.4.95 FPREM, FPREM1: Floating−Point Partial RemainderFPREM ; D9 F8 [8086,FPU] FPREM1 ; D9 F5 [386,FPU]

These instructions both produce the remainder obtained by dividing ST0 by ST1. This iscalculated, notionally, by dividing ST0 by ST1, rounding the result to an integer, multiplying byST1 again, and computing the value which would need to be added back on to the result to get backto the original value in ST0.

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The two instructions differ in the way the notional round−to−integer operation is performed.FPREM does it by rounding towards zero, so that the remainder it returns always has the same signas the original value in ST0; FPREM1 does it by rounding to the nearest integer, so that theremainder always has at most half the magnitude of ST1.

Both instructions calculate partial remainders, meaning that they may not manage to provide thefinal result, but might leave intermediate results in ST0 instead. If this happens, they will set the C2flag in the FPU status word; therefore, to calculate a remainder, you should repeatedly executeFPREM or FPREM1 until C2 becomes clear.

B.4.96 FRNDINT: Floating−Point Round to IntegerFRNDINT ; D9 FC [8086,FPU]

FRNDINT rounds the contents of ST0 to an integer, according to the current rounding mode set inthe FPU control word, and stores the result back in ST0.

B.4.97 FSAVE, FRSTOR: Save/Restore Floating−Point StateFSAVE mem ; 9B DD /6 [8086,FPU] FNSAVE mem ; DD /6 [8086,FPU]

FRSTOR mem ; DD /4 [8086,FPU]

FSAVE saves the entire floating−point unit state, including all the information saved by FSTENV(section B.4.104) plus the contents of all the registers, to a 94 or 108 byte area of memory(depending on the CPU mode). FRSTOR restores the floating−point state from the same area ofmemory.

FNSAVE does the same as FSAVE, without first waiting for pending floating−point exceptions toclear.

B.4.98 FSCALE: Scale Floating−Point Value by Power of TwoFSCALE ; D9 FD [8086,FPU]

FSCALE scales a number by a power of two: it rounds ST1 towards zero to obtain an integer, thenmultiplies ST0 by two to the power of that integer, and stores the result in ST0.

B.4.99 FSETPM: Set Protected ModeFSETPM ; DB E4 [286,FPU]

This instruction initialises protected mode on the 287 floating−point coprocessor. It is onlymeaningful on that processor: the 387 and above treat the instruction as a no−operation.

B.4.100 FSIN , FSINCOS: Sine and CosineFSIN ; D9 FE [386,FPU] FSINCOS ; D9 FB [386,FPU]

FSIN calculates the sine of ST0 (in radians) and stores the result in ST0. FSINCOS does the same,but then pushes the cosine of the same value on the register stack, so that the sine ends up in ST1and the cosine in ST0. FSINCOS is faster than executing FSIN and FCOS (see section B.4.74) insuccession.

The absolute value of ST0 must be less than 2**63.

B.4.101 FSQRT: Floating−Point Square RootFSQRT ; D9 FA [8086,FPU]

FSQRT calculates the square root of ST0 and stores the result in ST0.

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B.4.102 FST, FSTP: Floating−Point StoreFST mem32 ; D9 /2 [8086,FPU] FST mem64 ; DD /2 [8086,FPU] FST fpureg ; DD D0+r [8086,FPU]

FSTP mem32 ; D9 /3 [8086,FPU] FSTP mem64 ; DD /3 [8086,FPU] FSTP mem80 ; DB /7 [8086,FPU] FSTP fpureg ; DD D8+r [8086,FPU]

FST stores the value in ST0 into the given memory location or other FPU register. FSTP does thesame, but then pops the register stack.

B.4.103 FSTCW: Store Floating−Point Control WordFSTCW mem16 ; 9B D9 /7 [8086,FPU] FNSTCW mem16 ; D9 /7 [8086,FPU]

FSTCW stores the FPU control word (governing things like the rounding mode, the precision, andthe exception masks) into a 2−byte memory area. See also FLDCW (section B.4.90).

FNSTCW does the same thing as FSTCW, without first waiting for pending floating−point exceptionsto clear.

B.4.104 FSTENV: Store Floating−Point EnvironmentFSTENV mem ; 9B D9 /6 [8086,FPU] FNSTENV mem ; D9 /6 [8086,FPU]

FSTENV stores the FPU operating environment (control word, status word, tag word, instructionpointer, data pointer and last opcode) into memory. The memory area is 14 or 28 bytes long,depending on the CPU mode at the time. See also FLDENV (section B.4.91).

FNSTENV does the same thing as FSTENV, without first waiting for pending floating−pointexceptions to clear.

B.4.105 FSTSW: Store Floating−Point Status WordFSTSW mem16 ; 9B DD /7 [8086,FPU] FSTSW AX ; 9B DF E0 [286,FPU]

FNSTSW mem16 ; DD /7 [8086,FPU] FNSTSW AX ; DF E0 [286,FPU]

FSTSW stores the FPU status word into AX or into a 2−byte memory area.

FNSTSW does the same thing as FSTSW, without first waiting for pending floating−point exceptionsto clear.

B.4.106 FSUB, FSUBP, FSUBR, FSUBRP: Floating−Point SubtractFSUB mem32 ; D8 /4 [8086,FPU] FSUB mem64 ; DC /4 [8086,FPU]

FSUB fpureg ; D8 E0+r [8086,FPU] FSUB ST0,fpureg ; D8 E0+r [8086,FPU]

FSUB TO fpureg ; DC E8+r [8086,FPU] FSUB fpureg,ST0 ; DC E8+r [8086,FPU]

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FSUBR mem32 ; D8 /5 [8086,FPU] FSUBR mem64 ; DC /5 [8086,FPU]

FSUBR fpureg ; D8 E8+r [8086,FPU] FSUBR ST0,fpureg ; D8 E8+r [8086,FPU]

FSUBR TO fpureg ; DC E0+r [8086,FPU] FSUBR fpureg,ST0 ; DC E0+r [8086,FPU]

FSUBP fpureg ; DE E8+r [8086,FPU] FSUBP fpureg,ST0 ; DE E8+r [8086,FPU]

FSUBRP fpureg ; DE E0+r [8086,FPU] FSUBRP fpureg,ST0 ; DE E0+r [8086,FPU]

• FSUB subtracts the given operand from ST0 and stores the result back in ST0, unless the TOqualifier is given, in which case it subtracts ST0 from the given operand and stores the result inthe operand.

• FSUBR does the same thing, but does the subtraction the other way up: so if TO is not given, itsubtracts ST0 from the given operand and stores the result in ST0, whereas if TO is given itsubtracts its operand from ST0 and stores the result in the operand.

• FSUBP operates like FSUB TO, but pops the register stack once it has finished.

• FSUBRP operates like FSUBR TO, but pops the register stack once it has finished.

B.4.107 FTST: Test ST0 Against ZeroFTST ; D9 E4 [8086,FPU]

FTST compares ST0 with zero and sets the FPU flags accordingly. ST0 is treated as the left−handside of the comparison, so that a ‘less−than’ result is generated if ST0 is negative.

B.4.108 FUCOMxx: Floating−Point Unordered CompareFUCOM fpureg ; DD E0+r [386,FPU] FUCOM ST0,fpureg ; DD E0+r [386,FPU]

FUCOMP fpureg ; DD E8+r [386,FPU] FUCOMP ST0,fpureg ; DD E8+r [386,FPU]

FUCOMPP ; DA E9 [386,FPU]

FUCOMI fpureg ; DB E8+r [P6,FPU] FUCOMI ST0,fpureg ; DB E8+r [P6,FPU]

FUCOMIP fpureg ; DF E8+r [P6,FPU] FUCOMIP ST0,fpureg ; DF E8+r [P6,FPU]

• FUCOM compares ST0 with the given operand, and sets the FPU flags accordingly. ST0 istreated as the left−hand side of the comparison, so that the carry flag is set (for a ‘less−than’result) if ST0 is less than the given operand.

• FUCOMP does the same as FUCOM, but pops the register stack afterwards. FUCOMPP comparesST0 with ST1 and then pops the register stack twice.

• FUCOMI and FUCOMIP work like the corresponding forms of FUCOM and FUCOMP, but writetheir results directly to the CPU flags register rather than the FPU status word, so they can beimmediately followed by conditional jump or conditional move instructions.

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The FUCOM instructions differ from the FCOM instructions (section B.4.73) only in the way theyhandle quiet NaNs: FUCOM will handle them silently and set the condition code flags to an‘unordered’ result, whereas FCOM will generate an exception.

B.4.109 FXAM: Examine Class of Value in ST0

FXAM ; D9 E5 [8086,FPU]

FXAM sets the FPU flags C3, C2 and C0 depending on the type of value stored in ST0:

Register contents Flags

Unsupported format 000 NaN 001 Finite number 010 Infinity 011 Zero 100 Empty register 101 Denormal 110

Additionally, the C1 flag is set to the sign of the number.

B.4.110 FXCH: Floating−Point ExchangeFXCH ; D9 C9 [8086,FPU] FXCH fpureg ; D9 C8+r [8086,FPU] FXCH fpureg,ST0 ; D9 C8+r [8086,FPU] FXCH ST0,fpureg ; D9 C8+r [8086,FPU]

FXCH exchanges ST0 with a given FPU register. The no−operand form exchanges ST0 with ST1.

B.4.111 FXRSTOR: Restore FP, MMX and SSE StateFXRSTOR memory ; 0F AE /1 [P6,SSE,FPU]

The FXRSTOR instruction reloads the FPU, MMX and SSE state (environment and registers), fromthe 512 byte memory area defined by the source operand. This data should have been written by aprevious FXSAVE.

B.4.112 FXSAVE: Store FP, MMX and SSE StateFXSAVE memory ; 0F AE /0 [P6,SSE,FPU]

FXSAVEThe FXSAVE instruction writes the current FPU, MMX and SSE technology states(environment and registers), to the 512 byte memory area defined by the destination operand. Itdoes this without checking for pending unmasked floating−point exceptions (similar to theoperation of FNSAVE).

Unlike the FSAVE/FNSAVE instructions, the processor retains the contents of the FPU, MMX andSSE state in the processor after the state has been saved. This instruction has been optimised tomaximize floating−point save performance.

B.4.113 FXTRACT: Extract Exponent and SignificandFXTRACT ; D9 F4 [8086,FPU]

FXTRACT separates the number in ST0 into its exponent and significand (mantissa), stores theexponent back into ST0, and then pushes the significand on the register stack (so that thesignificand ends up in ST0, and the exponent in ST1).

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B.4.114 FYL2X, FYL2XP1: Compute Y times Log2(X) or Log2(X+1)FYL2X ; D9 F1 [8086,FPU] FYL2XP1 ; D9 F9 [8086,FPU]

FYL2X multiplies ST1 by the base−2 logarithm of ST0, stores the result in ST1, and pops theregister stack (so that the result ends up in ST0). ST0 must be non−zero and positive.

FYL2XP1 works the same way, but replacing the base−2 log of ST0 with that of ST0 plus one.This time, ST0 must have magnitude no greater than 1 minus half the square root of two.

B.4.115 HLT: Halt ProcessorHLT ; F4 [8086,PRIV]

HLT puts the processor into a halted state, where it will perform no more operations until restartedby an interrupt or a reset.

On the 286 and later processors, this is a privileged instruction.

B.4.116 IBTS : Insert Bit StringIBTS r/m16,reg16 ; o16 0F A7 /r [386,UNDOC] IBTS r/m32,reg32 ; o32 0F A7 /r [386,UNDOC]

The implied operation of this instruction is:

IBTS r/m16,AX,CL,reg16 IBTS r/m32,EAX,CL,reg32

Writes a bit string from the source operand to the destination. CL indicates the number of bits to becopied, from the low bits of the source. (E)AX indicates the low order bit offset in the destinationthat is written to. For example, if CL is set to 4 and AX (for 16−bit code) is set to 5, bits 0−3 of srcwill be copied to bits 5−8 of dst . This instruction is very poorly documented, and I have beenunable to find any official source of documentation on it.

IBTS is supported only on the early Intel 386s, and conflicts with the opcodes for CMPXCHG486(on early Intel 486s). NASM supports it only for completeness. Its counterpart is XBTS (see sectionB.4.332).

B.4.117 IDIV : Signed Integer DivideIDIV r/m8 ; F6 /7 [8086] IDIV r/m16 ; o16 F7 /7 [8086] IDIV r/m32 ; o32 F7 /7 [386]

IDIV performs signed integer division. The explicit operand provided is the divisor; the dividendand destination operands are implicit, in the following way:

• For IDIV r/m8 , AX is divided by the given operand; the quotient is stored in AL and theremainder in AH.

• For IDIV r/m16 , DX:AX is divided by the given operand; the quotient is stored in AX and theremainder in DX.

• For IDIV r/m32 , EDX:EAX is divided by the given operand; the quotient is stored in EAX andthe remainder in EDX.

Unsigned integer division is performed by the DIV instruction: see section B.4.59.

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B.4.118 IMUL: Signed Integer MultiplyIMUL r/m8 ; F6 /5 [8086] IMUL r/m16 ; o16 F7 /5 [8086] IMUL r/m32 ; o32 F7 /5 [386]

IMUL reg16,r/m16 ; o16 0F AF /r [386] IMUL reg32,r/m32 ; o32 0F AF /r [386]

IMUL reg16,imm8 ; o16 6B /r ib [186] IMUL reg16,imm16 ; o16 69 /r iw [186] IMUL reg32,imm8 ; o32 6B /r ib [386] IMUL reg32,imm32 ; o32 69 /r id [386]

IMUL reg16,r/m16,imm8 ; o16 6B /r ib [186] IMUL reg16,r/m16,imm16 ; o16 69 /r iw [186] IMUL reg32,r/m32,imm8 ; o32 6B /r ib [386] IMUL reg32,r/m32,imm32 ; o32 69 /r id [386]

IMUL performs signed integer multiplication. For the single−operand form, the other operand anddestination are implicit, in the following way:

• For IMUL r/m8 , AL is multiplied by the given operand; the product is stored in AX.

• For IMUL r/m16 , AX is multiplied by the given operand; the product is stored in DX:AX.

• For IMUL r/m32 , EAX is multiplied by the given operand; the product is stored in EDX:EAX.

The two−operand form multiplies its two operands and stores the result in the destination (first)operand. The three−operand form multiplies its last two operands and stores the result in the firstoperand.

The two−operand form with an immediate second operand is in fact a shorthand for thethree−operand form, as can be seen by examining the opcode descriptions: in the two−operandform, the code /r takes both its register and r/m parts from the same operand (the first one).

In the forms with an 8−bit immediate operand and another longer source operand, the immediateoperand is considered to be signed, and is sign−extended to the length of the other source operand.In these cases, the BYTE qualifier is necessary to force NASM to generate this form of theinstruction.

Unsigned integer multiplication is performed by the MUL instruction: see section B.4.184.

B.4.119 IN : Input from I/O PortIN AL,imm8 ; E4 ib [8086] IN AX,imm8 ; o16 E5 ib [8086] IN EAX,imm8 ; o32 E5 ib [386] IN AL,DX ; EC [8086] IN AX,DX ; o16 ED [8086] IN EAX,DX ; o32 ED [386]

IN reads a byte, word or doubleword from the specified I/O port, and stores it in the givendestination register. The port number may be specified as an immediate value if it is between 0 and255, and otherwise must be stored in DX. See also OUT (section B.4.194).

B.4.120 INC: Increment IntegerINC reg16 ; o16 40+r [8086] INC reg32 ; o32 40+r [386] INC r/m8 ; FE /0 [8086]

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INC r/m16 ; o16 FF /0 [8086] INC r/m32 ; o32 FF /0 [386]

INC adds 1 to its operand. It does not affect the carry flag: to affect the carry flag, useADD something,1 (see section B.4.3). INC affects all the other flags according to the result.

This instruction can be used with a LOCK prefix to allow atomic execution.

See also DEC (section B.4.58).

B.4.121 INSB, INSW, INSD: Input String from I/O PortINSB ; 6C [186] INSW ; o16 6D [186] INSD ; o32 6D [386]

INSB inputs a byte from the I/O port specified in DX and stores it at [ES:DI] or [ES:EDI] . Itthen increments or decrements (depending on the direction flag: increments if the flag is clear,decrements if it is set) DI or EDI .

The register used is DI if the address size is 16 bits, and EDI if it is 32 bits. If you need to use anaddress size not equal to the current BITS setting, you can use an explicit a16 or a32 prefix.

Segment override prefixes have no effect for this instruction: the use of ES for the load from [DI]or [EDI] cannot be overridden.

INSW and INSD work in the same way, but they input a word or a doubleword instead of a byte,and increment or decrement the addressing register by 2 or 4 instead of 1.

The REP prefix may be used to repeat the instruction CX (or ECX – again, the address size chooseswhich) times.

See also OUTSB, OUTSW and OUTSD (section B.4.195).

B.4.122 INT : Software InterruptINT imm8 ; CD ib [8086]

INT causes a software interrupt through a specified vector number from 0 to 255.

The code generated by the INT instruction is always two bytes long: although there are short formsfor some INT instructions, NASM does not generate them when it sees the INT mnemonic. Inorder to generate single−byte breakpoint instructions, use the INT3 or INT1 instructions (seesection B.4.123) instead.

B.4.123 INT3 , INT1 , ICEBP, INT01 : BreakpointsINT1 ; F1 [P6] ICEBP ; F1 [P6] INT01 ; F1 [P6]

INT3 ; CC [8086] INT03 ; CC [8086]

INT1 and INT3 are short one−byte forms of the instructions INT 1 and INT 3 (see sectionB.4.122). They perform a similar function to their longer counterparts, but take up less code space.They are used as breakpoints by debuggers.

• INT1 , and its alternative synonyms INT01 and ICEBP, is an instruction used by in−circuitemulators (ICEs). It is present, though not documented, on some processors down to the 286, butis only documented for the Pentium Pro. INT3 is the instruction normally used as a breakpointby debuggers.

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• INT3 , and its synonym INT03 , is not precisely equivalent to INT 3 : the short form, since it isdesigned to be used as a breakpoint, bypasses the normal IOPL checks in virtual−8086 mode,and also does not go through interrupt redirection.

B.4.124 INTO: Interrupt if OverflowINTO ; CE [8086]

INTO performs an INT 4 software interrupt (see section B.4.122) if and only if the overflow flagis set.

B.4.125 INVD: Invalidate Internal CachesINVD ; 0F 08 [486]

INVD invalidates and empties the processor’s internal caches, and causes the processor to instructexternal caches to do the same. It does not write the contents of the caches back to memory first:any modified data held in the caches will be lost. To write the data back first, use WBINVD (sectionB.4.328).

B.4.126 INVLPG: Invalidate TLB EntryINVLPG mem ; 0F 01 /7 [486]

INVLPG invalidates the translation lookahead buffer (TLB) entry associated with the suppliedmemory address.

B.4.127 IRET , IRETW, IRETD: Return from InterruptIRET ; CF [8086] IRETW ; o16 CF [8086] IRETD ; o32 CF [386]

IRET returns from an interrupt (hardware or software) by means of popping IP (or EIP ), CS andthe flags off the stack and then continuing execution from the new CS:IP .

IRETW pops IP , CS and the flags as 2 bytes each, taking 6 bytes off the stack in total. IRETD popsEIP as 4 bytes, pops a further 4 bytes of which the top two are discarded and the bottom two gointo CS, and pops the flags as 4 bytes as well, taking 12 bytes off the stack.

IRET is a shorthand for either IRETW or IRETD, depending on the default BITS setting at the time.

B.4.128 Jcc : Conditional BranchJcc imm ; 70+cc rb [8086] Jcc NEAR imm ; 0F 80+cc rw/rd [386]

The conditional jump instructions execute a near (same segment) jump if and only if theirconditions are satisfied. For example, JNZ jumps only if the zero flag is not set.

The ordinary form of the instructions has only a 128−byte range; the NEAR form is a 386 extensionto the instruction set, and can span the full size of a segment. NASM will not override your choiceof jump instruction: if you want Jcc NEAR , you have to use the NEAR keyword.

The SHORT keyword is allowed on the first form of the instruction, for clarity, but is not necessary.

For details of the condition codes, see section B.2.2.

B.4.129 JCXZ, JECXZ: Jump if CX/ECX ZeroJCXZ imm ; a16 E3 rb [8086] JECXZ imm ; a32 E3 rb [386]

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JCXZ performs a short jump (with maximum range 128 bytes) if and only if the contents of the CXregister is 0. JECXZ does the same thing, but with ECX.

B.4.130 JMP: JumpJMP imm ; E9 rw/rd [8086] JMP SHORT imm ; EB rb [8086] JMP imm:imm16 ; o16 EA iw iw [8086] JMP imm:imm32 ; o32 EA id iw [386] JMP FAR mem ; o16 FF /5 [8086] JMP FAR mem32 ; o32 FF /5 [386] JMP r/m16 ; o16 FF /4 [8086] JMP r/m32 ; o32 FF /4 [386]

JMP jumps to a given address. The address may be specified as an absolute segment and offset, oras a relative jump within the current segment.

JMP SHORT imm has a maximum range of 128 bytes, since the displacement is specified as only8 bits, but takes up less code space. NASM does not choose when to generate JMP SHORT for you:you must explicitly code SHORT every time you want a short jump.

You can choose between the two immediate far jump forms (JMP imm:imm ) by the use of theWORD and DWORD keywords: JMP WORD 0x1234:0x5678 ) orJMP DWORD 0x1234:0x56789abc .

The JMP FAR mem forms execute a far jump by loading the destination address out of memory.The address loaded consists of 16 or 32 bits of offset (depending on the operand size), and 16 bits ofsegment. The operand size may be overridden using JMP WORD FAR mem orJMP DWORD FAR mem.

The JMP r/m forms execute a near jump (within the same segment), loading the destinationaddress out of memory or out of a register. The keyword NEAR may be specified, for clarity, inthese forms, but is not necessary. Again, operand size can be overridden using JMP WORD mem orJMP DWORD mem.

As a convenience, NASM does not require you to jump to a far symbol by coding the cumbersomeJMP SEG routine:routine , but instead allows the easier synonym JMP FAR routine .

The CALL r/m forms given above are near calls; NASM will accept the NEAR keyword (e.g.CALL NEAR [address] ), even though it is not strictly necessary.

B.4.131 LAHF: Load AH from FlagsLAHF ; 9F [8086]

LAHF sets the AH register according to the contents of the low byte of the flags word.

The operation of LAHF is:

AH <−− SF:ZF:0:AF:0:PF:1:CF

See also SAHF (section B.4.282).

B.4.132 LAR: Load Access RightsLAR reg16,r/m16 ; o16 0F 02 /r [286,PRIV] LAR reg32,r/m32 ; o32 0F 02 /r [286,PRIV]

LAR takes the segment selector specified by its source (second) operand, finds the correspondingsegment descriptor in the GDT or LDT, and loads the access−rights byte of the descriptor into itsdestination (first) operand.

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B.4.133 LDMXCSR: Load Streaming SIMD Extension Control/StatusLDMXCSR mem32 ; 0F AE /2 [KATMAI,SSE]

LDMXCSR loads 32−bits of data from the specified memory location into the MXCSR control/statusregister. MXCSR is used to enable masked/unmasked exception handling, to set rounding modes, toset flush−to−zero mode, and to view exception status flags.

For details of the MXCSR register, see the Intel processor docs.

See also STMXCSR (section B.4.302

B.4.134 LDS, LES, LFS, LGS, LSS: Load Far PointerLDS reg16,mem ; o16 C5 /r [8086] LDS reg32,mem ; o32 C5 /r [386]

LES reg16,mem ; o16 C4 /r [8086] LES reg32,mem ; o32 C4 /r [386]

LFS reg16,mem ; o16 0F B4 /r [386] LFS reg32,mem ; o32 0F B4 /r [386]

LGS reg16,mem ; o16 0F B5 /r [386] LGS reg32,mem ; o32 0F B5 /r [386]

LSS reg16,mem ; o16 0F B2 /r [386] LSS reg32,mem ; o32 0F B2 /r [386]

These instructions load an entire far pointer (16 or 32 bits of offset, plus 16 bits of segment) out ofmemory in one go. LDS, for example, loads 16 or 32 bits from the given memory address into thegiven register (depending on the size of the register), then loads the next 16 bits from memory intoDS. LES, LFS, LGS and LSS work in the same way but use the other segment registers.

B.4.135 LEA: Load Effective AddressLEA reg16,mem ; o16 8D /r [8086] LEA reg32,mem ; o32 8D /r [386]

LEA, despite its syntax, does not access memory. It calculates the effective address specified by itssecond operand as if it were going to load or store data from it, but instead it stores the calculatedaddress into the register specified by its first operand. This can be used to perform quite complexcalculations (e.g. LEA EAX,[EBX+ECX*4+100] ) in one instruction.

LEA, despite being a purely arithmetic instruction which accesses no memory, still requires squarebrackets around its second operand, as if it were a memory reference.

The size of the calculation is the current address size, and the size that the result is stored as is thecurrent operand size. If the address and operand size are not the same, then if the addressing modewas 32−bits, the low 16−bits are stored, and if the address was 16−bits, it is zero−extended to32−bits before storing.

B.4.136 LEAVE: Destroy Stack FrameLEAVE ; C9 [186]

LEAVE destroys a stack frame of the form created by the ENTER instruction (see section B.4.65). Itis functionally equivalent to MOV ESP,EBP followed by POP EBP (or MOV SP,BP followed byPOP BP in 16−bit mode).

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B.4.137 LFENCE: Load FenceLFENCE ; 0F AE /5 [WILLAMETTE,SSE2]

LFENCE performs a serialising operation on all loads from memory that were issued before theLFENCE instruction. This guarantees that all memory reads before the LFENCE instruction arevisible before any reads after the LFENCE instruction.

LFENCE is ordered respective to other LFENCE instruction, MFENCE, any memory read and anyother serialising instruction (such as CPUID).

Weakly ordered memory types can be used to achieve higher processor performance through suchtechniques as out−of−order issue and speculative reads. The degree to which a consumer of datarecognizes or knows that the data is weakly ordered varies among applications and may beunknown to the producer of this data. The LFENCE instruction provides a performance−efficientway of ensuring load ordering between routines that produce weakly−ordered results and routinesthat consume that data.

LFENCE uses the following ModRM encoding:

Mod (7:6) = 11B Reg/Opcode (5:3) = 101B R/M (2:0) = 000B

All other ModRM encodings are defined to be reserved, and use of these encodings risksincompatibility with future processors.

See also SFENCE (section B.4.288) and MFENCE (section B.4.151).

B.4.138 LGDT, LIDT , LLDT: Load Descriptor TablesLGDT mem ; 0F 01 /2 [286,PRIV] LIDT mem ; 0F 01 /3 [286,PRIV] LLDT r/m16 ; 0F 00 /2 [286,PRIV]

LGDT and LIDT both take a 6−byte memory area as an operand: they load a 32−bit linear addressand a 16−bit size limit from that area (in the opposite order) into the GDTR (global descriptor tableregister) or IDTR (interrupt descriptor table register). These are the only instructions which directlyuse linear addresses, rather than segment/offset pairs.

LLDT takes a segment selector as an operand. The processor looks up that selector in the GDT andstores the limit and base address given there into the LDTR (local descriptor table register).

See also SGDT, SIDT and SLDT (section B.4.289).

B.4.139 LMSW: Load/Store Machine Status WordLMSW r/m16 ; 0F 01 /6 [286,PRIV]

LMSW loads the bottom four bits of the source operand into the bottom four bits of the CR0 controlregister (or the Machine Status Word, on 286 processors). See also SMSW (section B.4.296).

B.4.140 LOADALL, LOADALL286: Load Processor StateLOADALL ; 0F 07 [386,UNDOC] LOADALL286 ; 0F 05 [286,UNDOC]

This instruction, in its two different−opcode forms, is apparently supported on most 286 processors,some 386 and possibly some 486. The opcode differs between the 286 and the 386.

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The function of the instruction is to load all information relating to the state of the processor out of ablock of memory: on the 286, this block is located implicitly at absolute address 0x800 , and on the386 and 486 it is at [ES:EDI] .

B.4.141 LODSB, LODSW, LODSD: Load from StringLODSB ; AC [8086] LODSW ; o16 AD [8086] LODSD ; o32 AD [386]

LODSB loads a byte from [DS:SI] or [DS:ESI] into AL. It then increments or decrements(depending on the direction flag: increments if the flag is clear, decrements if it is set) SI or ESI .

The register used is SI if the address size is 16 bits, and ESI if it is 32 bits. If you need to use anaddress size not equal to the current BITS setting, you can use an explicit a16 or a32 prefix.

The segment register used to load from [SI] or [ESI] can be overridden by using a segmentregister name as a prefix (for example, ES LODSB).

LODSW and LODSD work in the same way, but they load a word or a doubleword instead of a byte,and increment or decrement the addressing registers by 2 or 4 instead of 1.

B.4.142 LOOP, LOOPE, LOOPZ, LOOPNE, LOOPNZ: Loop with CounterLOOP imm ; E2 rb [8086] LOOP imm,CX ; a16 E2 rb [8086] LOOP imm,ECX ; a32 E2 rb [386]

LOOPE imm ; E1 rb [8086] LOOPE imm,CX ; a16 E1 rb [8086] LOOPE imm,ECX ; a32 E1 rb [386] LOOPZ imm ; E1 rb [8086] LOOPZ imm,CX ; a16 E1 rb [8086] LOOPZ imm,ECX ; a32 E1 rb [386]

LOOPNE imm ; E0 rb [8086] LOOPNE imm,CX ; a16 E0 rb [8086] LOOPNE imm,ECX ; a32 E0 rb [386] LOOPNZ imm ; E0 rb [8086] LOOPNZ imm,CX ; a16 E0 rb [8086] LOOPNZ imm,ECX ; a32 E0 rb [386]

LOOP decrements its counter register (either CX or ECX – if one is not specified explicitly, theBITS setting dictates which is used) by one, and if the counter does not become zero as a result ofthis operation, it jumps to the given label. The jump has a range of 128 bytes.

LOOPE (or its synonym LOOPZ) adds the additional condition that it only jumps if the counter isnonzero and the zero flag is set. Similarly, LOOPNE (and LOOPNZ) jumps only if the counter isnonzero and the zero flag is clear.

B.4.143 LSL: Load Segment LimitLSL reg16,r/m16 ; o16 0F 03 /r [286,PRIV] LSL reg32,r/m32 ; o32 0F 03 /r [286,PRIV]

LSL is given a segment selector in its source (second) operand; it computes the segment limit valueby loading the segment limit field from the associated segment descriptor in the GDT or LDT. (Thisinvolves shifting left by 12 bits if the segment limit is page−granular, and not if it is byte−granular;so you end up with a byte limit in either case.) The segment limit obtained is then loaded into thedestination (first) operand.

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B.4.144 LTR: Load Task RegisterLTR r/m16 ; 0F 00 /3 [286,PRIV]

LTR looks up the segment base and limit in the GDT or LDT descriptor specified by the segmentselector given as its operand, and loads them into the Task Register.

B.4.145 MASKMOVDQU: Byte Mask WriteMASKMOVDQU xmm1,xmm2 ; 66 0F F7 /r [WILLAMETTE,SSE2]

MASKMOVDQU stores data from xmm1 to the location specified by ES:(E)DI . The size of thestore depends on the address−size attribute. The most significant bit in each byte of the maskregister xmm2 is used to selectively write the data (0 = no write, 1 = write) on a per−byte basis.

B.4.146 MASKMOVQ: Byte Mask WriteMASKMOVQ mm1,mm2 ; 0F F7 /r [KATMAI,MMX]

MASKMOVQ stores data from mm1 to the location specified by ES:(E)DI . The size of the storedepends on the address−size attribute. The most significant bit in each byte of the mask registermm2 is used to selectively write the data (0 = no write, 1 = write) on a per−byte basis.

B.4.147 MAXPD: Return Packed Double−Precision FP MaximumMAXPD xmm1,xmm2/m128 ; 66 0F 5F /r [WILLAMETTE,SSE2]

MAXPD performs a SIMD compare of the packed double−precision FP numbers from xmm1 andxmm2/mem, and stores the maximum values of each pair of values in xmm1. If the values beingcompared are both zeroes, source2 (xmm2/m128) would be returned. If source2 (xmm2/m128) is anSNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN isnot returned).

B.4.148 MAXPS: Return Packed Single−Precision FP MaximumMAXPS xmm1,xmm2/m128 ; 0F 5F /r [KATMAI,SSE]

MAXPS performs a SIMD compare of the packed single−precision FP numbers from xmm1 andxmm2/mem, and stores the maximum values of each pair of values in xmm1. If the values beingcompared are both zeroes, source2 (xmm2/m128) would be returned. If source2 (xmm2/m128) is anSNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN isnot returned).

B.4.149 MAXSD: Return Scalar Double−Precision FP MaximumMAXSD xmm1,xmm2/m64 ; F2 0F 5F /r [WILLAMETTE,SSE2]

MAXSD compares the low−order double−precision FP numbers from xmm1 and xmm2/mem, andstores the maximum value in xmm1. If the values being compared are both zeroes, source2(xmm2/m64) would be returned. If source2 (xmm2/m64) is an SNaN, this SNaN is forwardedunchanged to the destination (i.e., a QNaN version of the SNaN is not returned). The highquadword of the destination is left unchanged.

B.4.150 MAXSS: Return Scalar Single−Precision FP MaximumMAXSS xmm1,xmm2/m32 ; F3 0F 5F /r [KATMAI,SSE]

MAXSS compares the low−order single−precision FP numbers from xmm1 and xmm2/mem, andstores the maximum value in xmm1. If the values being compared are both zeroes, source2(xmm2/m32) would be returned. If source2 (xmm2/m32) is an SNaN, this SNaN is forwardedunchanged to the destination (i.e., a QNaN version of the SNaN is not returned). The high threedoublewords of the destination are left unchanged.

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B.4.151 MFENCE: Memory FenceMFENCE ; 0F AE /6 [WILLAMETTE,SSE2]

MFENCE performs a serialising operation on all loads from memory and writes to memory that wereissued before the MFENCE instruction. This guarantees that all memory reads and writes before theMFENCE instruction are completed before any reads and writes after the MFENCE instruction.

MFENCE is ordered respective to other MFENCE instructions, LFENCE, SFENCE, any memory readand any other serialising instruction (such as CPUID).

Weakly ordered memory types can be used to achieve higher processor performance through suchtechniques as out−of−order issue, speculative reads, write−combining, and write−collapsing. Thedegree to which a consumer of data recognizes or knows that the data is weakly ordered variesamong applications and may be unknown to the producer of this data. The MFENCE instructionprovides a performance−efficient way of ensuring load and store ordering between routines thatproduce weakly−ordered results and routines that consume that data.

MFENCE uses the following ModRM encoding:



See also LFENCE (section B.4.137) and SFENCE (section B.4.288).

B.4.152 MINPD: Return Packed Double−Precision FP MinimumMINPD xmm1,xmm2/m128 ; 66 0F 5D /r [WILLAMETTE,SSE2]

MINPD performs a SIMD compare of the packed double−precision FP numbers from xmm1 andxmm2/mem, and stores the minimum values of each pair of values in xmm1. If the values beingcompared are both zeroes, source2 (xmm2/m128) would be returned. If source2 (xmm2/m128) is anSNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN isnot returned).

B.4.153 MINPS: Return Packed Single−Precision FP MinimumMINPS xmm1,xmm2/m128 ; 0F 5D /r [KATMAI,SSE]

MINPS performs a SIMD compare of the packed single−precision FP numbers from xmm1 andxmm2/mem, and stores the minimum values of each pair of values in xmm1. If the values beingcompared are both zeroes, source2 (xmm2/m128) would be returned. If source2 (xmm2/m128) is anSNaN, this SNaN is forwarded unchanged to the destination (i.e., a QNaN version of the SNaN isnot returned).

B.4.154 MINSD: Return Scalar Double−Precision FP MinimumMINSD xmm1,xmm2/m64 ; F2 0F 5D /r [WILLAMETTE,SSE2]

MINSD compares the low−order double−precision FP numbers from xmm1 and xmm2/mem, andstores the minimum value in xmm1. If the values being compared are both zeroes, source2(xmm2/m64) would be returned. If source2 (xmm2/m64) is an SNaN, this SNaN is forwardedunchanged to the destination (i.e., a QNaN version of the SNaN is not returned). The highquadword of the destination is left unchanged.

B.4.155 MINSS: Return Scalar Single−Precision FP MinimumMINSS xmm1,xmm2/m32 ; F3 0F 5D /r [KATMAI,SSE]

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MINSS compares the low−order single−precision FP numbers from xmm1 and xmm2/mem, andstores the minimum value in xmm1. If the values being compared are both zeroes, source2(xmm2/m32) would be returned. If source2 (xmm2/m32) is an SNaN, this SNaN is forwardedunchanged to the destination (i.e., a QNaN version of the SNaN is not returned). The high threedoublewords of the destination are left unchanged.

B.4.156 MOV: Move DataMOV r/m8,reg8 ; 88 /r [8086] MOV r/m16,reg16 ; o16 89 /r [8086] MOV r/m32,reg32 ; o32 89 /r [386] MOV reg8,r/m8 ; 8A /r [8086] MOV reg16,r/m16 ; o16 8B /r [8086] MOV reg32,r/m32 ; o32 8B /r [386]

MOV reg8,imm8 ; B0+r ib [8086] MOV reg16,imm16 ; o16 B8+r iw [8086] MOV reg32,imm32 ; o32 B8+r id [386] MOV r/m8,imm8 ; C6 /0 ib [8086] MOV r/m16,imm16 ; o16 C7 /0 iw [8086] MOV r/m32,imm32 ; o32 C7 /0 id [386]

MOV AL,memoffs8 ; A0 ow/od [8086] MOV AX,memoffs16 ; o16 A1 ow/od [8086] MOV EAX,memoffs32 ; o32 A1 ow/od [386] MOV memoffs8,AL ; A2 ow/od [8086] MOV memoffs16,AX ; o16 A3 ow/od [8086] MOV memoffs32,EAX ; o32 A3 ow/od [386]

MOV r/m16,segreg ; o16 8C /r [8086] MOV r/m32,segreg ; o32 8C /r [386] MOV segreg,r/m16 ; o16 8E /r [8086] MOV segreg,r/m32 ; o32 8E /r [386]

MOV reg32,CR0/2/3/4 ; 0F 20 /r [386] MOV reg32,DR0/1/2/3/6/7 ; 0F 21 /r [386] MOV reg32,TR3/4/5/6/7 ; 0F 24 /r [386] MOV CR0/2/3/4,reg32 ; 0F 22 /r [386] MOV DR0/1/2/3/6/7,reg32 ; 0F 23 /r [386] MOV TR3/4/5/6/7,reg32 ; 0F 26 /r [386]

MOV copies the contents of its source (second) operand into its destination (first) operand.

In all forms of the MOV instruction, the two operands are the same size, except for moving betweena segment register and an r/m32 operand. These instructions are treated exactly like thecorresponding 16−bit equivalent (so that, for example, MOV DS,EAX functions identically toMOV DS,AX but saves a prefix when in 32−bit mode), except that when a segment register ismoved into a 32−bit destination, the top two bytes of the result are undefined.

MOV may not use CS as a destination.

CR4 is only a supported register on the Pentium and above.

Test registers are supported on 386/486 processors and on some non−Intel Pentium class processors.

B.4.157 MOVAPD: Move Aligned Packed Double−Precision FP ValuesMOVAPD xmm1,xmm2/mem128 ; 66 0F 28 /r [WILLAMETTE,SSE2] MOVAPD xmm1/mem128,xmm2 ; 66 0F 29 /r [WILLAMETTE,SSE2]

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MOVAPD moves a double quadword containing 2 packed double−precision FP values from thesource operand to the destination. When the source or destination operand is a memory location, itmust be aligned on a 16−byte boundary.

To move data in and out of memory locations that are not known to be on 16−byte boundaries, usethe MOVUPD instruction (section B.4.182).

B.4.158 MOVAPS: Move Aligned Packed Single−Precision FP ValuesMOVAPS xmm1,xmm2/mem128 ; 0F 28 /r [KATMAI,SSE] MOVAPS xmm1/mem128,xmm2 ; 0F 29 /r [KATMAI,SSE]

MOVAPS moves a double quadword containing 4 packed single−precision FP values from thesource operand to the destination. When the source or destination operand is a memory location, itmust be aligned on a 16−byte boundary.

To move data in and out of memory locations that are not known to be on 16−byte boundaries, usethe MOVUPS instruction (section B.4.183).

B.4.159 MOVD: Move Doubleword to/from MMX RegisterMOVD mm,r/m32 ; 0F 6E /r [PENT,MMX] MOVD r/m32,mm ; 0F 7E /r [PENT,MMX] MOVD xmm,r/m32 ; 66 0F 6E /r [WILLAMETTE,SSE2] MOVD r/m32,xmm ; 66 0F 7E /r [WILLAMETTE,SSE2]

MOVD copies 32 bits from its source (second) operand into its destination (first) operand. When thedestination is a 64−bit MMX register or a 128−bit XMM register, the input value is zero−extended tofill the destination register.

B.4.160 MOVDQ2Q: Move Quadword from XMM to MMX register.MOVDQ2Q mm,xmm ; F2 OF D6 /r [WILLAMETTE,SSE2]

MOVDQ2Q moves the low quadword from the source operand to the destination operand.

B.4.161 MOVDQA: Move Aligned Double QuadwordMOVDQA xmm1,xmm2/m128 ; 66 OF 6F /r [WILLAMETTE,SSE2] MOVDQA xmm1/m128,xmm2 ; 66 OF 7F /r [WILLAMETTE,SSE2]

MOVDQA moves a double quadword from the source operand to the destination operand. When thesource or destination operand is a memory location, it must be aligned to a 16−byte boundary.

To move a double quadword to or from unaligned memory locations, use the MOVDQU instruction(section B.4.162).

B.4.162 MOVDQU: Move Unaligned Double QuadwordMOVDQU xmm1,xmm2/m128 ; F3 OF 6F /r [WILLAMETTE,SSE2] MOVDQU xmm1/m128,xmm2 ; F3 OF 7F /r [WILLAMETTE,SSE2]

MOVDQU moves a double quadword from the source operand to the destination operand. When thesource or destination operand is a memory location, the memory may be unaligned.

To move a double quadword to or from known aligned memory locations, use the MOVDQAinstruction (section B.4.161).

B.4.163 MOVHLPS: Move Packed Single−Precision FP High to LowMOVHLPS xmm1,xmm2 ; OF 12 /r [KATMAI,SSE]

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MOVHLPS moves the two packed single−precision FP values from the high quadword of the sourceregister xmm2 to the low quadword of the destination register, xmm2. The upper quadword ofxmm1 is left unchanged.

The operation of this instruction is:

dst[0−63] := src[64−127], dst[64−127] remains unchanged.

B.4.164 MOVHPD: Move High Packed Double−Precision FPMOVHPD xmm,m64 ; 66 OF 16 /r [WILLAMETTE,SSE2] MOVHPD m64,xmm ; 66 OF 17 /r [WILLAMETTE,SSE2]

MOVHPD moves a double−precision FP value between the source and destination operands. One ofthe operands is a 64−bit memory location, the other is the high quadword of an XMM register.


mem[0−63] := xmm[64−127];

or

xmm[0−63] remains unchanged; xmm[64−127] := mem[0−63].

B.4.165 MOVHPS: Move High Packed Single−Precision FPMOVHPS xmm,m64 ; 0F 16 /r [KATMAI,SSE] MOVHPS m64,xmm ; 0F 17 /r [KATMAI,SSE]

MOVHPS moves two packed single−precision FP values between the source and destinationoperands. One of the operands is a 64−bit memory location, the other is the high quadword of anXMM register.


mem[0−63] := xmm[64−127];

or

xmm[0−63] remains unchanged; xmm[64−127] := mem[0−63].

B.4.166 MOVLHPS: Move Packed Single−Precision FP Low to HighMOVLHPS xmm1,xmm2 ; OF 16 /r [KATMAI,SSE]

MOVLHPS moves the two packed single−precision FP values from the low quadword of the sourceregister xmm2 to the high quadword of the destination register, xmm2. The low quadword of xmm1is left unchanged.


dst[0−63] remains unchanged; dst[64−127] := src[0−63].

B.4.167 MOVLPD: Move Low Packed Double−Precision FPMOVLPD xmm,m64 ; 66 OF 12 /r [WILLAMETTE,SSE2] MOVLPD m64,xmm ; 66 OF 13 /r [WILLAMETTE,SSE2]

MOVLPD moves a double−precision FP value between the source and destination operands. One ofthe operands is a 64−bit memory location, the other is the low quadword of an XMM register.

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mem(0−63) := xmm(0−63);

or

xmm(0−63) := mem(0−63); xmm(64−127) remains unchanged.

B.4.168 MOVLPS: Move Low Packed Single−Precision FPMOVLPS xmm,m64 ; OF 12 /r [KATMAI,SSE] MOVLPS m64,xmm ; OF 13 /r [KATMAI,SSE]

MOVLPS moves two packed single−precision FP values between the source and destinationoperands. One of the operands is a 64−bit memory location, the other is the low quadword of anXMM register.


mem(0−63) := xmm(0−63);

or

xmm(0−63) := mem(0−63); xmm(64−127) remains unchanged.

B.4.169 MOVMSKPD: Extract Packed Double−Precision FP Sign MaskMOVMSKPD reg32,xmm ; 66 0F 50 /r [WILLAMETTE,SSE2]

MOVMSKPD inserts a 2−bit mask in r32, formed of the most significant bits of eachdouble−precision FP number of the source operand.

B.4.170 MOVMSKPS: Extract Packed Single−Precision FP Sign MaskMOVMSKPS reg32,xmm ; 0F 50 /r [KATMAI,SSE]

MOVMSKPS inserts a 4−bit mask in r32, formed of the most significant bits of each single−precisionFP number of the source operand.

B.4.171 MOVNTDQ: Move Double Quadword Non TemporalMOVNTDQ m128,xmm ; 66 0F E7 /r [WILLAMETTE,SSE2]

MOVNTDQ moves the double quadword from the XMM source register to the destination memorylocation, using a non−temporal hint. This store instruction minimizes cache pollution.

B.4.172 MOVNTI: Move Doubleword Non TemporalMOVNTI m32,reg32 ; 0F C3 /r [WILLAMETTE,SSE2]

MOVNTI moves the doubleword in the source register to the destination memory location, using anon−temporal hint. This store instruction minimizes cache pollution.

B.4.173 MOVNTPD: Move Aligned Four Packed Single−Precision FP Values Non TemporalMOVNTPD m128,xmm ; 66 0F 2B /r [WILLAMETTE,SSE2]

MOVNTPD moves the double quadword from the XMM source register to the destination memorylocation, using a non−temporal hint. This store instruction minimizes cache pollution. The memorylocation must be aligned to a 16−byte boundary.

158

B.4.174 MOVNTPS: Move Aligned Four Packed Single−Precision FP Values Non TemporalMOVNTPS m128,xmm ; 0F 2B /r [KATMAI,SSE]

MOVNTPS moves the double quadword from the XMM source register to the destination memorylocation, using a non−temporal hint. This store instruction minimizes cache pollution. The memorylocation must be aligned to a 16−byte boundary.

B.4.175 MOVNTQ: Move Quadword Non TemporalMOVNTQ m64,mm ; 0F E7 /r [KATMAI,MMX]

MOVNTQ moves the quadword in the MMX source register to the destination memory location, usinga non−temporal hint. This store instruction minimizes cache pollution.

B.4.176 MOVQ: Move Quadword to/from MMX RegisterMOVQ mm1,mm2/m64 ; 0F 6F /r [PENT,MMX] MOVQ mm1/m64,mm2 ; 0F 7F /r [PENT,MMX]

MOVQ xmm1,xmm2/m64 ; F3 0F 7E /r [WILLAMETTE,SSE2] MOVQ xmm1/m64,xmm2 ; 66 0F D6 /r [WILLAMETTE,SSE2]

MOVQ copies 64 bits from its source (second) operand into its destination (first) operand. When thesource is an XMM register, the low quadword is moved. When the destination is an XMM register, thedestination is the low quadword, and the high quadword is cleared.

B.4.177 MOVQ2DQ: Move Quadword from MMX to XMM register.MOVQ2DQ xmm,mm ; F3 OF D6 /r [WILLAMETTE,SSE2]

MOVQ2DQ moves the quadword from the source operand to the low quadword of the destinationoperand, and clears the high quadword.

B.4.178 MOVSB, MOVSW, MOVSD: Move StringMOVSB ; A4 [8086] MOVSW ; o16 A5 [8086] MOVSD ; o32 A5 [386]

MOVSB copies the byte at [DS:SI] or [DS:ESI] to [ES:DI] or [ES:EDI] . It then incrementsor decrements (depending on the direction flag: increments if the flag is clear, decrements if it isset) SI and DI (or ESI and EDI ).

The registers used are SI and DI if the address size is 16 bits, and ESI and EDI if it is 32 bits. Ifyou need to use an address size not equal to the current BITS setting, you can use an explicit a16or a32 prefix.

The segment register used to load from [SI] or [ESI] can be overridden by using a segmentregister name as a prefix (for example, es movsb ). The use of ES for the store to [DI] or [EDI]cannot be overridden.

MOVSW and MOVSD work in the same way, but they copy a word or a doubleword instead of a byte,and increment or decrement the addressing registers by 2 or 4 instead of 1.


B.4.179 MOVSD: Move Scalar Double−Precision FP ValueMOVSD xmm1,xmm2/m64 ; F2 0F 10 /r [WILLAMETTE,SSE2] MOVSD xmm1/m64,xmm2 ; F2 0F 11 /r [WILLAMETTE,SSE2]

159

MOVSD moves a double−precision FP value from the source operand to the destination operand.When the source or destination is a register, the low−order FP value is read or written.

B.4.180 MOVSS: Move Scalar Single−Precision FP ValueMOVSS xmm1,xmm2/m32 ; F3 0F 10 /r [KATMAI,SSE] MOVSS xmm1/m32,xmm2 ; F3 0F 11 /r [KATMAI,SSE]

MOVSS moves a single−precision FP value from the source operand to the destination operand.When the source or destination is a register, the low−order FP value is read or written.

B.4.181 MOVSX, MOVZX: Move Data with Sign or Zero ExtendMOVSX reg16,r/m8 ; o16 0F BE /r [386] MOVSX reg32,r/m8 ; o32 0F BE /r [386] MOVSX reg32,r/m16 ; o32 0F BF /r [386]

MOVZX reg16,r/m8 ; o16 0F B6 /r [386] MOVZX reg32,r/m8 ; o32 0F B6 /r [386] MOVZX reg32,r/m16 ; o32 0F B7 /r [386]

MOVSX sign−extends its source (second) operand to the length of its destination (first) operand, andcopies the result into the destination operand. MOVZX does the same, but zero−extends rather thansign−extending.

B.4.182 MOVUPD: Move Unaligned Packed Double−Precision FP ValuesMOVUPD xmm1,xmm2/mem128 ; 66 0F 10 /r [WILLAMETTE,SSE2] MOVUPD xmm1/mem128,xmm2 ; 66 0F 11 /r [WILLAMETTE,SSE2]

MOVUPD moves a double quadword containing 2 packed double−precision FP values from thesource operand to the destination. This instruction makes no assumptions about alignment ofmemory operands.

To move data in and out of memory locations that are known to be on 16−byte boundaries, use theMOVAPD instruction (section B.4.157).

B.4.183 MOVUPS: Move Unaligned Packed Single−Precision FP ValuesMOVUPS xmm1,xmm2/mem128 ; 0F 10 /r [KATMAI,SSE] MOVUPS xmm1/mem128,xmm2 ; 0F 11 /r [KATMAI,SSE]

MOVUPS moves a double quadword containing 4 packed single−precision FP values from thesource operand to the destination. This instruction makes no assumptions about alignment ofmemory operands.

To move data in and out of memory locations that are known to be on 16−byte boundaries, use theMOVAPS instruction (section B.4.158).

B.4.184 MUL: Unsigned Integer MultiplyMUL r/m8 ; F6 /4 [8086] MUL r/m16 ; o16 F7 /4 [8086] MUL r/m32 ; o32 F7 /4 [386]

MUL performs unsigned integer multiplication. The other operand to the multiplication, and thedestination operand, are implicit, in the following way:

• For MUL r/m8 , AL is multiplied by the given operand; the product is stored in AX.

• For MUL r/m16 , AX is multiplied by the given operand; the product is stored in DX:AX.

160

• For MUL r/m32 , EAX is multiplied by the given operand; the product is stored in EDX:EAX.

Signed integer multiplication is performed by the IMUL instruction: see section B.4.118.

B.4.185 MULPD: Packed Single−FP MultiplyMULPD xmm1,xmm2/mem128 ; 66 0F 59 /r [WILLAMETTE,SSE2]

MULPD performs a SIMD multiply of the packed double−precision FP values in both operands, andstores the results in the destination register.

B.4.186 MULPS: Packed Single−FP MultiplyMULPS xmm1,xmm2/mem128 ; 0F 59 /r [KATMAI,SSE]

MULPS performs a SIMD multiply of the packed single−precision FP values in both operands, andstores the results in the destination register.

B.4.187 MULSD: Scalar Single−FP MultiplyMULSD xmm1,xmm2/mem32 ; F2 0F 59 /r [WILLAMETTE,SSE2]

MULSD multiplies the lowest double−precision FP values of both operands, and stores the result inthe low quadword of xmm1.

B.4.188 MULSS: Scalar Single−FP MultiplyMULSS xmm1,xmm2/mem32 ; F3 0F 59 /r [KATMAI,SSE]

MULSS multiplies the lowest single−precision FP values of both operands, and stores the result inthe low doubleword of xmm1.

B.4.189 NEG, NOT: Two’s and One’s ComplementNEG r/m8 ; F6 /3 [8086] NEG r/m16 ; o16 F7 /3 [8086] NEG r/m32 ; o32 F7 /3 [386]

NOT r/m8 ; F6 /2 [8086] NOT r/m16 ; o16 F7 /2 [8086] NOT r/m32 ; o32 F7 /2 [386]

NEG replaces the contents of its operand by the two’s complement negation (invert all the bits andthen add one) of the original value. NOT, similarly, performs one’s complement (inverts all the bits).

B.4.190 NOP: No OperationNOP ; 90 [8086]

NOP performs no operation. Its opcode is the same as that generated by XCHG AX,AX orXCHG EAX,EAX (depending on the processor mode; see section B.4.333).

B.4.191 OR: Bitwise OROR r/m8,reg8 ; 08 /r [8086] OR r/m16,reg16 ; o16 09 /r [8086] OR r/m32,reg32 ; o32 09 /r [386]

OR reg8,r/m8 ; 0A /r [8086] OR reg16,r/m16 ; o16 0B /r [8086] OR reg32,r/m32 ; o32 0B /r [386]

161

OR r/m8,imm8 ; 80 /1 ib [8086] OR r/m16,imm16 ; o16 81 /1 iw [8086] OR r/m32,imm32 ; o32 81 /1 id [386]

OR r/m16,imm8 ; o16 83 /1 ib [8086] OR r/m32,imm8 ; o32 83 /1 ib [386]

OR AL,imm8 ; 0C ib [8086] OR AX,imm16 ; o16 0D iw [8086] OR EAX,imm32 ; o32 0D id [386]

OR performs a bitwise OR operation between its two operands (i.e. each bit of the result is 1 if andonly if at least one of the corresponding bits of the two inputs was 1), and stores the result in thedestination (first) operand.


The MMX instruction POR (see section B.4.247) performs the same operation on the 64−bit MMXregisters.

B.4.192 ORPD: Bit−wise Logical OR of Double−Precision FP DataORPD xmm1,xmm2/m128 ; 66 0F 56 /r [WILLAMETTE,SSE2]

ORPD return a bit−wise logical OR between xmm1 and xmm2/mem, and stores the result in xmm1.If the source operand is a memory location, it must be aligned to a 16−byte boundary.

B.4.193 ORPS: Bit−wise Logical OR of Single−Precision FP DataORPS xmm1,xmm2/m128 ; 0F 56 /r [KATMAI,SSE]

ORPS return a bit−wise logical OR between xmm1 and xmm2/mem, and stores the result in xmm1.If the source operand is a memory location, it must be aligned to a 16−byte boundary.

B.4.194 OUT: Output Data to I/O PortOUT imm8,AL ; E6 ib [8086] OUT imm8,AX ; o16 E7 ib [8086] OUT imm8,EAX ; o32 E7 ib [386] OUT DX,AL ; EE [8086] OUT DX,AX ; o16 EF [8086] OUT DX,EAX ; o32 EF [386]

OUT writes the contents of the given source register to the specified I/O port. The port number maybe specified as an immediate value if it is between 0 and 255, and otherwise must be stored in DX.See also IN (section B.4.119).

B.4.195 OUTSB, OUTSW, OUTSD: Output String to I/O PortOUTSB ; 6E [186] OUTSW ; o16 6F [186] OUTSD ; o32 6F [386]

OUTSB loads a byte from [DS:SI] or [DS:ESI] and writes it to the I/O port specified in DX. Itthen increments or decrements (depending on the direction flag: increments if the flag is clear,decrements if it is set) SI or ESI .

The register used is SI if the address size is 16 bits, and ESI if it is 32 bits. If you need to use anaddress size not equal to the current BITS setting, you can use an explicit a16 or a32 prefix.

162

The segment register used to load from [SI] or [ESI] can be overridden by using a segmentregister name as a prefix (for example, es outsb ).

OUTSW and OUTSD work in the same way, but they output a word or a doubleword instead of abyte, and increment or decrement the addressing registers by 2 or 4 instead of 1.


B.4.196 PACKSSDW, PACKSSWB, PACKUSWB: Pack DataPACKSSDW mm1,mm2/m64 ; 0F 6B /r [PENT,MMX] PACKSSWB mm1,mm2/m64 ; 0F 63 /r [PENT,MMX] PACKUSWB mm1,mm2/m64 ; 0F 67 /r [PENT,MMX]

PACKSSDW xmm1,xmm2/m128 ; 66 0F 6B /r [WILLAMETTE,SSE2] PACKSSWB xmm1,xmm2/m128 ; 66 0F 63 /r [WILLAMETTE,SSE2] PACKUSWB xmm1,xmm2/m128 ; 66 0F 67 /r [WILLAMETTE,SSE2]

All these instructions start by combining the source and destination operands, and then splitting theresult in smaller sections which it then packs into the destination register. The MMX versions packtwo 64−bit operands into one 64−bit register, while the SSE versions pack two 128−bit operandsinto one 128−bit register.

• PACKSSWB splits the combined value into words, and then reduces the words to bytes, usingsigned saturation. It then packs the bytes into the destination register in the same order the wordswere in.

• PACKSSDW performs the same operation as PACKSSWB, except that it reduces doublewords towords, then packs them into the destination register.

• PACKUSWB performs the same operation as PACKSSWB, except that it uses unsigned saturationwhen reducing the size of the elements.

To perform signed saturation on a number, it is replaced by the largest signed number (7FFFh or7Fh) that will fit, and if it is too small it is replaced by the smallest signed number (8000h or 80h )that will fit. To perform unsigned saturation, the input is treated as unsigned, and the input isreplaced by the largest unsigned number that will fit.

B.4.197 PADDB, PADDW, PADDD: Add Packed IntegersPADDB mm1,mm2/m64 ; 0F FC /r [PENT,MMX] PADDW mm1,mm2/m64 ; 0F FD /r [PENT,MMX] PADDD mm1,mm2/m64 ; 0F FE /r [PENT,MMX]

PADDB xmm1,xmm2/m128 ; 66 0F FC /r [WILLAMETTE,SSE2] PADDW xmm1,xmm2/m128 ; 66 0F FD /r [WILLAMETTE,SSE2] PADDD xmm1,xmm2/m128 ; 66 0F FE /r [WILLAMETTE,SSE2]

PADDx performs packed addition of the two operands, storing the result in the destination (first)operand.

• PADDB treats the operands as packed bytes, and adds each byte individually;

• PADDW treats the operands as packed words;

• PADDD treats its operands as packed doublewords.

When an individual result is too large to fit in its destination, it is wrapped around and the low bitsare stored, with the carry bit discarded.

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B.4.198 PADDQ: Add Packed Quadword IntegersPADDQ mm1,mm2/m64 ; 0F D4 /r [PENT,MMX]

PADDQ xmm1,xmm2/m128 ; 66 0F D4 /r [WILLAMETTE,SSE2]

PADDQ adds the quadwords in the source and destination operands, and stores the result in thedestination register.

When an individual result is too large to fit in its destination, it is wrapped around and the low bitsare stored, with the carry bit discarded.

B.4.199 PADDSB, PADDSW: Add Packed Signed Integers With SaturationPADDSB mm1,mm2/m64 ; 0F EC /r [PENT,MMX] PADDSW mm1,mm2/m64 ; 0F ED /r [PENT,MMX]

PADDSB xmm1,xmm2/m128 ; 66 0F EC /r [WILLAMETTE,SSE2] PADDSW xmm1,xmm2/m128 ; 66 0F ED /r [WILLAMETTE,SSE2]

PADDSx performs packed addition of the two operands, storing the result in the destination (first)operand. PADDSB treats the operands as packed bytes, and adds each byte individually; andPADDSW treats the operands as packed words.

When an individual result is too large to fit in its destination, a saturated value is stored. Theresulting value is the value with the largest magnitude of the same sign as the result which will fit inthe available space.

B.4.200 PADDSIW: MMX Packed Addition to Implicit DestinationPADDSIW mmxreg,r/m64 ; 0F 51 /r [CYRIX,MMX]

PADDSIW, specific to the Cyrix extensions to the MMX instruction set, performs the same functionas PADDSW, except that the result is placed in an implied register.

To work out the implied register, invert the lowest bit in the register number. SoPADDSIW MM0,MM2 would put the result in MM1, but PADDSIW MM1,MM2 would put the resultin MM0.

B.4.201 PADDUSB, PADDUSW: Add Packed Unsigned Integers With SaturationPADDUSB mm1,mm2/m64 ; 0F DC /r [PENT,MMX] PADDUSW mm1,mm2/m64 ; 0F DD /r [PENT,MMX]

PADDUSB xmm1,xmm2/m128 ; 66 0F DC /r [WILLAMETTE,SSE2] PADDUSW xmm1,xmm2/m128 ; 66 0F DD /r [WILLAMETTE,SSE2]

PADDUSx performs packed addition of the two operands, storing the result in the destination (first)operand. PADDUSB treats the operands as packed bytes, and adds each byte individually; andPADDUSW treats the operands as packed words.

When an individual result is too large to fit in its destination, a saturated value is stored. Theresulting value is the maximum value that will fit in the available space.

B.4.202 PAND, PANDN: MMX Bitwise AND and AND−NOTPAND mm1,mm2/m64 ; 0F DB /r [PENT,MMX] PANDN mm1,mm2/m64 ; 0F DF /r [PENT,MMX]

PAND xmm1,xmm2/m128 ; 66 0F DB /r [WILLAMETTE,SSE2] PANDN xmm1,xmm2/m128 ; 66 0F DF /r [WILLAMETTE,SSE2]

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PAND performs a bitwise AND operation between its two operands (i.e. each bit of the result is 1 ifand only if the corresponding bits of the two inputs were both 1), and stores the result in thedestination (first) operand.

PANDN performs the same operation, but performs a one’s complement operation on the destination(first) operand first.

B.4.203 PAUSE: Spin Loop HintPAUSE ; F3 90 [WILLAMETTE,SSE2]

PAUSE provides a hint to the processor that the following code is a spin loop. This improvesprocessor performance by bypassing possible memory order violations. On older processors, thisinstruction operates as a NOP.

B.4.204 PAVEB: MMX Packed AveragePAVEB mmxreg,r/m64 ; 0F 50 /r [CYRIX,MMX]

PAVEB, specific to the Cyrix MMX extensions, treats its two operands as vectors of eight unsignedbytes, and calculates the average of the corresponding bytes in the operands. The resulting vector ofeight averages is stored in the first operand.

This opcode maps to MOVMSKPS r32, xmm on processors that support the SSE instruction set.

B.4.205 PAVGB PAVGW: Average Packed IntegersPAVGB mm1,mm2/m64 ; 0F E0 /r [KATMAI,MMX] PAVGW mm1,mm2/m64 ; 0F E3 /r [KATMAI,MMX,SM]

PAVGB xmm1,xmm2/m128 ; 66 0F E0 /r [WILLAMETTE,SSE2] PAVGW xmm1,xmm2/m128 ; 66 0F E3 /r [WILLAMETTE,SSE2]

PAVGB and PAVGW add the unsigned data elements of the source operand to the unsigned dataelements of the destination register, then adds 1 to the temporary results. The results of the add arethen each independently right−shifted by one bit position. The high order bits of each element arefilled with the carry bits of the corresponding sum.

• PAVGB operates on packed unsigned bytes, and

• PAVGW operates on packed unsigned words.

B.4.206 PAVGUSB: Average of unsigned packed 8−bit valuesPAVGUSB mm1,mm2/m64 ; 0F 0F /r BF [PENT,3DNOW]

PAVGUSB adds the unsigned data elements of the source operand to the unsigned data elements ofthe destination register, then adds 1 to the temporary results. The results of the add are then eachindependently right−shifted by one bit position. The high order bits of each element are filled withthe carry bits of the corresponding sum.

This instruction performs exactly the same operations as the PAVGB MMX instruction (sectionB.4.205).

B.4.207 PCMPxx: Compare Packed Integers.PCMPEQB mm1,mm2/m64 ; 0F 74 /r [PENT,MMX] PCMPEQW mm1,mm2/m64 ; 0F 75 /r [PENT,MMX] PCMPEQD mm1,mm2/m64 ; 0F 76 /r [PENT,MMX]

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PCMPGTB mm1,mm2/m64 ; 0F 64 /r [PENT,MMX] PCMPGTW mm1,mm2/m64 ; 0F 65 /r [PENT,MMX] PCMPGTD mm1,mm2/m64 ; 0F 66 /r [PENT,MMX]

PCMPEQB xmm1,xmm2/m128 ; 66 0F 74 /r [WILLAMETTE,SSE2] PCMPEQW xmm1,xmm2/m128 ; 66 0F 75 /r [WILLAMETTE,SSE2] PCMPEQD xmm1,xmm2/m128 ; 66 0F 76 /r [WILLAMETTE,SSE2]

PCMPGTB xmm1,xmm2/m128 ; 66 0F 64 /r [WILLAMETTE,SSE2] PCMPGTW xmm1,xmm2/m128 ; 66 0F 65 /r [WILLAMETTE,SSE2] PCMPGTD xmm1,xmm2/m128 ; 66 0F 66 /r [WILLAMETTE,SSE2]

The PCMPxx instructions all treat their operands as vectors of bytes, words, or doublewords;corresponding elements of the source and destination are compared, and the corresponding elementof the destination (first) operand is set to all zeros or all ones depending on the result of thecomparison.

• PCMPxxB treats the operands as vectors of bytes;

• PCMPxxW treats the operands as vectors of words;

• PCMPxxD treats the operands as vectors of doublewords;

• PCMPEQx sets the corresponding element of the destination operand to all ones if the twoelements compared are equal;

• PCMPGTx sets the destination element to all ones if the element of the first (destination) operandis greater (treated as a signed integer) than that of the second (source) operand.

B.4.208 PDISTIB : MMX Packed Distance and Accumulate with Implied RegisterPDISTIB mm,m64 ; 0F 54 /r [CYRIX,MMX]

PDISTIB , specific to the Cyrix MMX extensions, treats its two input operands as vectors of eightunsigned bytes. For each byte position, it finds the absolute difference between the bytes in thatposition in the two input operands, and adds that value to the byte in the same position in theimplied output register. The addition is saturated to an unsigned byte in the same way as PADDUSB.

To work out the implied register, invert the lowest bit in the register number. SoPDISTIB MM0,M64 would put the result in MM1, but PDISTIB MM1,M64 would put the resultin MM0.

Note that PDISTIB cannot take a register as its second source operand.

Operation:

dstI[0−7] := dstI[0−7] + ABS(src0[0−7] − src1[0−7]), dstI[8−15] := dstI[8−15] + ABS(src0[8−15] − src1[8−15]), ....... ....... dstI[56−63] := dstI[56−63] + ABS(src0[56−63] − src1[56−63]).

B.4.209 PEXTRW: Extract WordPEXTRW reg32,mm,imm8 ; 0F C5 /r ib [KATMAI,MMX] PEXTRW reg32,xmm,imm8 ; 66 0F C5 /r ib [WILLAMETTE,SSE2]

PEXTRW moves the word in the source register (second operand) that is pointed to by the countoperand (third operand), into the lower half of a 32−bit general purpose register. The upper half ofthe register is cleared to all 0s.

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When the source operand is an MMX register, the two least significant bits of the count specify thesource word. When it is an SSE register, the three least significant bits specify the word location.

B.4.210 PF2ID : Packed Single−Precision FP to Integer ConvertPF2ID mm1,mm2/m64 ; 0F 0F /r 1D [PENT,3DNOW]

PF2ID converts two single−precision FP values in the source operand to signed 32−bit integers,using truncation, and stores them in the destination operand. Source values that are outside therange supported by the destination are saturated to the largest absolute value of the same sign.

B.4.211 PF2IW: Packed Single−Precision FP to Integer Word ConvertPF2IW mm1,mm2/m64 ; 0F 0F /r 1C [PENT,3DNOW]

PF2IW converts two single−precision FP values in the source operand to signed 16−bit integers,using truncation, and stores them in the destination operand. Source values that are outside therange supported by the destination are saturated to the largest absolute value of the same sign.

• In the K6−2 and K6−III, the 16−bit value is zero−extended to 32−bits before storing.

• In the K6−2+, K6−III+ and Athlon processors, the value is sign−extended to 32−bits beforestoring.

B.4.212 PFACC: Packed Single−Precision FP AccumulatePFACC mm1,mm2/m64 ; 0F 0F /r AE [PENT,3DNOW]

PFACC adds the two single−precision FP values from the destination operand together, then addsthe two single−precision FP values from the source operand, and places the results in the low andhigh doublewords of the destination operand.

The operation is:

dst[0−31] := dst[0−31] + dst[32−63], dst[32−63] := src[0−31] + src[32−63].

B.4.213 PFADD: Packed Single−Precision FP AdditionPFADD mm1,mm2/m64 ; 0F 0F /r 9E [PENT,3DNOW]

PFADD performs addition on each of two packed single−precision FP value pairs.

dst[0−31] := dst[0−31] + src[0−31], dst[32−63] := dst[32−63] + src[32−63].

B.4.214 PFCMPxx: Packed Single−Precision FP Compare PFCMPEQ mm1,mm2/m64 ; 0F 0F /r B0 [PENT,3DNOW] PFCMPGE mm1,mm2/m64 ; 0F 0F /r 90 [PENT,3DNOW] PFCMPGT mm1,mm2/m64 ; 0F 0F /r A0 [PENT,3DNOW]

The PFCMPxx instructions compare the packed single−point FP values in the source anddestination operands, and set the destination according to the result. If the condition is true, thedestination is set to all 1s, otherwise it’s set to all 0s.

• PFCMPEQ tests whether dst == src;

• PFCMPGE tests whether dst >= src;

• PFCMPGT tests whether dst > src.

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B.4.215 PFMAX: Packed Single−Precision FP MaximumPFMAX mm1,mm2/m64 ; 0F 0F /r A4 [PENT,3DNOW]

PFMAX returns the higher of each pair of single−precision FP values. If the higher value is zero, it isreturned as positive zero.

B.4.216 PFMIN: Packed Single−Precision FP MinimumPFMIN mm1,mm2/m64 ; 0F 0F /r 94 [PENT,3DNOW]

PFMIN returns the lower of each pair of single−precision FP values. If the lower value is zero, it isreturned as positive zero.

B.4.217 PFMUL: Packed Single−Precision FP MultiplyPFMUL mm1,mm2/m64 ; 0F 0F /r B4 [PENT,3DNOW]

PFMUL returns the product of each pair of single−precision FP values.

dst[0−31] := dst[0−31] * src[0−31], dst[32−63] := dst[32−63] * src[32−63].

B.4.218 PFNACC: Packed Single−Precision FP Negative AccumulatePFNACC mm1,mm2/m64 ; 0F 0F /r 8A [PENT,3DNOW]

PFNACC performs a negative accumulate of the two single−precision FP values in the source anddestination registers. The result of the accumulate from the destination register is stored in the lowdoubleword of the destination, and the result of the source accumulate is stored in the highdoubleword of the destination register.

The operation is:

dst[0−31] := dst[0−31] − dst[32−63], dst[32−63] := src[0−31] − src[32−63].

B.4.219 PFPNACC: Packed Single−Precision FP Mixed AccumulatePFPNACC mm1,mm2/m64 ; 0F 0F /r 8E [PENT,3DNOW]

PFPNACC performs a positive accumulate of the two single−precision FP values in the sourceregister and a negative accumulate of the destination register. The result of the accumulate from thedestination register is stored in the low doubleword of the destination, and the result of the sourceaccumulate is stored in the high doubleword of the destination register.

The operation is:

dst[0−31] := dst[0−31] − dst[32−63], dst[32−63] := src[0−31] + src[32−63].

B.4.220 PFRCP: Packed Single−Precision FP Reciprocal ApproximationPFRCP mm1,mm2/m64 ; 0F 0F /r 96 [PENT,3DNOW]

PFRCP performs a low precision estimate of the reciprocal of the low−order single−precision FPvalue in the source operand, storing the result in both halves of the destination register. The result isaccurate to 14 bits.

For higher precision reciprocals, this instruction should be followed by two more instructions:PFRCPIT1 (section B.4.221) and PFRCPIT2 (section B.4.221). This will result in a 24−bitaccuracy. For more details, see the AMD 3DNow! technology manual.

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B.4.221 PFRCPIT1: Packed Single−Precision FP Reciprocal, First Iteration StepPFRCPIT1 mm1,mm2/m64 ; 0F 0F /r A6 [PENT,3DNOW]

PFRCPIT1 performs the first intermediate step in the calculation of the reciprocal of asingle−precision FP value. The first source value (mm1 is the original value, and the second sourcevalue (mm2/m64 is the result of a PFRCP instruction.

For the final step in a reciprocal, returning the full 24−bit accuracy of a single−precision FP value,see PFRCPIT2 (section B.4.222). For more details, see the AMD 3DNow! technology manual.

B.4.222 PFRCPIT2: Packed Single−Precision FP Reciprocal/ Reciprocal Square Root, SecondIteration StepPFRCPIT2 mm1,mm2/m64 ; 0F 0F /r B6 [PENT,3DNOW]

PFRCPIT2 performs the second and final intermediate step in the calculation of a reciprocal orreciprocal square root, refining the values returned by the PFRCP and PFRSQRT instructions,respectively.

The first source value (mm1) is the output of either a PFRCPIT1 or a PFRSQIT1 instruction, andthe second source is the output of either the PFRCP or the PFRSQRT instruction. For more details,see the AMD 3DNow! technology manual.

B.4.223 PFRSQIT1: Packed Single−Precision FP Reciprocal Square Root, First Iteration StepPFRSQIT1 mm1,mm2/m64 ; 0F 0F /r A7 [PENT,3DNOW]

PFRSQIT1 performs the first intermediate step in the calculation of the reciprocal square root of asingle−precision FP value. The first source value (mm1 is the square of the result of a PFRSQRTinstruction, and the second source value (mm2/m64 is the original value.

For the final step in a calculation, returning the full 24−bit accuracy of a single−precision FP value,see PFRCPIT2 (section B.4.222). For more details, see the AMD 3DNow! technology manual.

B.4.224 PFRSQRT: Packed Single−Precision FP Reciprocal Square Root ApproximationPFRSQRT mm1,mm2/m64 ; 0F 0F /r 97 [PENT,3DNOW]

PFRSQRT performs a low precision estimate of the reciprocal square root of the low−ordersingle−precision FP value in the source operand, storing the result in both halves of the destinationregister. The result is accurate to 15 bits.

For higher precision reciprocals, this instruction should be followed by two more instructions:PFRSQIT1 (section B.4.223) and PFRCPIT2 (section B.4.221). This will result in a 24−bitaccuracy. For more details, see the AMD 3DNow! technology manual.

B.4.225 PFSUB: Packed Single−Precision FP SubtractPFSUB mm1,mm2/m64 ; 0F 0F /r 9A [PENT,3DNOW]

PFSUB subtracts the single−precision FP values in the source from those in the destination, andstores the result in the destination operand.

dst[0−31] := dst[0−31] − src[0−31], dst[32−63] := dst[32−63] − src[32−63].

B.4.226 PFSUBR: Packed Single−Precision FP Reverse SubtractPFSUBR mm1,mm2/m64 ; 0F 0F /r AA [PENT,3DNOW]

PFSUBR subtracts the single−precision FP values in the destination from those in the source, andstores the result in the destination operand.

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dst[0−31] := src[0−31] − dst[0−31], dst[32−63] := src[32−63] − dst[32−63].

B.4.227 PI2FD : Packed Doubleword Integer to Single−Precision FP ConvertPI2FD mm1,mm2/m64 ; 0F 0F /r 0D [PENT,3DNOW]

PF2ID converts two signed 32−bit integers in the source operand to single−precision FP values,using truncation of significant digits, and stores them in the destination operand.

B.4.228 PF2IW: Packed Word Integer to Single−Precision FP ConvertPI2FW mm1,mm2/m64 ; 0F 0F /r 0C [PENT,3DNOW]

PF2IW converts two signed 16−bit integers in the source operand to single−precision FP values,and stores them in the destination operand. The input values are in the low word of eachdoubleword.

B.4.229 PINSRW: Insert WordPINSRW mm,r16/r32/m16,imm8 ;0F C4 /r ib [KATMAI,MMX] PINSRW xmm,r16/r32/m16,imm8 ;66 0F C4 /r ib [WILLAMETTE,SSE2]

PINSRW loads a word from a 16−bit register (or the low half of a 32−bit register), or from memory,and loads it to the word position in the destination register, pointed at by the count operand (thirdoperand). If the destination is an MMX register, the low two bits of the count byte are used, if it is anXMM register the low 3 bits are used. The insertion is done in such a way that the other words fromthe destination register are left untouched.

B.4.230 PMACHRIW: Packed Multiply and Accumulate with RoundingPMACHRIW mm,m64 ; 0F 5E /r [CYRIX,MMX]

PMACHRIW takes two packed 16−bit integer inputs, multiplies the values in the inputs, rounds onbit 15 of each result, then adds bits 15−30 of each result to the corresponding position of the implieddestination register.


dstI[0−15] := dstI[0−15] + (mm[0−15] *m64[0−15] + 0x00004000)[15−30], dstI[16−31] := dstI[16−31] + (mm[16−31]*m64[16−31] + 0x00004000)[15−30], dstI[32−47] := dstI[32−47] + (mm[32−47]*m64[32−47] + 0x00004000)[15−30], dstI[48−63] := dstI[48−63] + (mm[48−63]*m64[48−63] + 0x00004000)[15−30].

Note that PMACHRIW cannot take a register as its second source operand.

B.4.231 PMADDWD: MMX Packed Multiply and AddPMADDWD mm1,mm2/m64 ; 0F F5 /r [PENT,MMX] PMADDWD xmm1,xmm2/m128 ; 66 0F F5 /r [WILLAMETTE,SSE2]

PMADDWD treats its two inputs as vectors of signed words. It multiplies corresponding elements ofthe two operands, giving doubleword results. These are then added together in pairs and stored inthe destination operand.


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dst[0−31] := (dst[0−15] * src[0−15]) + (dst[16−31] * src[16−31]); dst[32−63] := (dst[32−47] * src[32−47]) + (dst[48−63] * src[48−63]);

The following apply to the SSE version of the instruction:

dst[64−95] := (dst[64−79] * src[64−79]) + (dst[80−95] * src[80−95]); dst[96−127] := (dst[96−111] * src[96−111]) + (dst[112−127] * src[112−127]).

B.4.232 PMAGW: MMX Packed MagnitudePMAGW mm1,mm2/m64 ; 0F 52 /r [CYRIX,MMX]

PMAGW, specific to the Cyrix MMX extensions, treats both its operands as vectors of four signedwords. It compares the absolute values of the words in corresponding positions, and sets each wordof the destination (first) operand to whichever of the two words in that position had the largerabsolute value.

B.4.233 PMAXSW: Packed Signed Integer Word MaximumPMAXSW mm1,mm2/m64 ; 0F EE /r [KATMAI,MMX] PMAXSW xmm1,xmm2/m128 ; 66 0F EE /r [WILLAMETTE,SSE2]

PMAXSW compares each pair of words in the two source operands, and for each pair it stores themaximum value in the destination register.

B.4.234 PMAXUB: Packed Unsigned Integer Byte MaximumPMAXUB mm1,mm2/m64 ; 0F DE /r [KATMAI,MMX] PMAXUB xmm1,xmm2/m128 ; 66 0F DE /r [WILLAMETTE,SSE2]

PMAXUB compares each pair of bytes in the two source operands, and for each pair it stores themaximum value in the destination register.

B.4.235 PMINSW: Packed Signed Integer Word MinimumPMINSW mm1,mm2/m64 ; 0F EA /r [KATMAI,MMX] PMINSW xmm1,xmm2/m128 ; 66 0F EA /r [WILLAMETTE,SSE2]

PMINSW compares each pair of words in the two source operands, and for each pair it stores theminimum value in the destination register.

B.4.236 PMINUB: Packed Unsigned Integer Byte MinimumPMINUB mm1,mm2/m64 ; 0F DA /r [KATMAI,MMX] PMINUB xmm1,xmm2/m128 ; 66 0F DA /r [WILLAMETTE,SSE2]

PMINUB compares each pair of bytes in the two source operands, and for each pair it stores theminimum value in the destination register.

B.4.237 PMOVMSKB: Move Byte Mask To IntegerPMOVMSKB reg32,mm ; 0F D7 /r [KATMAI,MMX] PMOVMSKB reg32,xmm ; 66 0F D7 /r [WILLAMETTE,SSE2]

PMOVMSKB returns an 8−bit or 16−bit mask formed of the most significant bits of each byte ofsource operand (8−bits for an MMX register, 16−bits for an XMM register).

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B.4.238 PMULHRWC, PMULHRIW: Multiply Packed 16−bit Integers With Rounding, and StoreHigh WordPMULHRWC mm1,mm2/m64 ; 0F 59 /r [CYRIX,MMX] PMULHRIW mm1,mm2/m64 ; 0F 5D /r [CYRIX,MMX]

These instructions take two packed 16−bit integer inputs, multiply the values in the inputs, round onbit 15 of each result, then store bits 15−30 of each result to the corresponding position of thedestination register.

• For PMULHRWC, the destination is the first source operand.

• For PMULHRIW, the destination is an implied register (worked out as described for PADDSIW(section B.4.200)).


dst[0−15] := (src1[0−15] *src2[0−15] + 0x00004000)[15−30] dst[16−31] := (src1[16−31]*src2[16−31] + 0x00004000)[15−30] dst[32−47] := (src1[32−47]*src2[32−47] + 0x00004000)[15−30] dst[48−63] := (src1[48−63]*src2[48−63] + 0x00004000)[15−30]

See also PMULHRWA (section B.4.239) for a 3DNow! version of this instruction.

B.4.239 PMULHRWA: Multiply Packed 16−bit Integers With Rounding, and Store High WordPMULHRWA mm1,mm2/m64 ; 0F 0F /r B7 [PENT,3DNOW]

PMULHRWA takes two packed 16−bit integer inputs, multiplies the values in the inputs, rounds onbit 16 of each result, then stores bits 16−31 of each result to the corresponding position of thedestination register.


dst[0−15] := (src1[0−15] *src2[0−15] + 0x00008000)[16−31]; dst[16−31] := (src1[16−31]*src2[16−31] + 0x00008000)[16−31]; dst[32−47] := (src1[32−47]*src2[32−47] + 0x00008000)[16−31]; dst[48−63] := (src1[48−63]*src2[48−63] + 0x00008000)[16−31].

See also PMULHRWC (section B.4.238) for a Cyrix version of this instruction.

B.4.240 PMULHUW: Multiply Packed 16−bit Integers, and Store High WordPMULHUW mm1,mm2/m64 ; 0F E4 /r [KATMAI,MMX] PMULHUW xmm1,xmm2/m128 ; 66 0F E4 /r [WILLAMETTE,SSE2]

PMULHUW takes two packed unsigned 16−bit integer inputs, multiplies the values in the inputs, thenstores bits 16−31 of each result to the corresponding position of the destination register.

B.4.241 PMULHW, PMULLW: Multiply Packed 16−bit Integers, and StorePMULHW mm1,mm2/m64 ; 0F E5 /r [PENT,MMX] PMULLW mm1,mm2/m64 ; 0F D5 /r [PENT,MMX]

PMULHW xmm1,xmm2/m128 ; 66 0F E5 /r [WILLAMETTE,SSE2] PMULLW xmm1,xmm2/m128 ; 66 0F D5 /r [WILLAMETTE,SSE2]

PMULxW takes two packed unsigned 16−bit integer inputs, and multiplies the values in the inputs,forming doubleword results.

• PMULHW then stores the top 16 bits of each doubleword in the destination (first) operand;

• PMULLW stores the bottom 16 bits of each doubleword in the destination operand.

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B.4.242 PMULUDQ: Multiply Packed Unsigned 32−bit Integers, and Store.PMULUDQ mm1,mm2/m64 ; 0F F4 /r [WILLAMETTE,SSE2] PMULUDQ xmm1,xmm2/m128 ; 66 0F F4 /r [WILLAMETTE,SSE2]

PMULUDQ takes two packed unsigned 32−bit integer inputs, and multiplies the values in the inputs,forming quadword results. The source is either an unsigned doubleword in the low doubleword of a64−bit operand, or it’s two unsigned doublewords in the first and third doublewords of a 128−bitoperand. This produces either one or two 64−bit results, which are stored in the respectivequadword locations of the destination register.

The operation is:

dst[0−63] := dst[0−31] * src[0−31]; dst[64−127] := dst[64−95] * src[64−95].

B.4.243 PMVccZB: MMX Packed Conditional MovePMVZB mmxreg,mem64 ; 0F 58 /r [CYRIX,MMX] PMVNZB mmxreg,mem64 ; 0F 5A /r [CYRIX,MMX] PMVLZB mmxreg,mem64 ; 0F 5B /r [CYRIX,MMX] PMVGEZB mmxreg,mem64 ; 0F 5C /r [CYRIX,MMX]

These instructions, specific to the Cyrix MMX extensions, perform parallel conditional moves. Thetwo input operands are treated as vectors of eight bytes. Each byte of the destination (first) operandis either written from the corresponding byte of the source (second) operand, or left alone,depending on the value of the byte in the implied operand (specified in the same way as PADDSIW,in section B.4.200).

• PMVZB performs each move if the corresponding byte in the implied operand is zero;

• PMVNZB moves if the byte is non−zero;

• PMVLZB moves if the byte is less than zero;

• PMVGEZB moves if the byte is greater than or equal to zero.

Note that these instructions cannot take a register as their second source operand.

B.4.244 POP: Pop Data from StackPOP reg16 ; o16 58+r [8086] POP reg32 ; o32 58+r [386]

POP r/m16 ; o16 8F /0 [8086] POP r/m32 ; o32 8F /0 [386]

POP CS ; 0F [8086,UNDOC] POP DS ; 1F [8086] POP ES ; 07 [8086] POP SS ; 17 [8086] POP FS ; 0F A1 [386] POP GS ; 0F A9 [386]

POP loads a value from the stack (from [SS:SP] or [SS:ESP] ) and then increments the stackpointer.

The address−size attribute of the instruction determines whether SP or ESP is used as the stackpointer: to deliberately override the default given by the BITS setting, you can use an a16 or a32prefix.

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The operand−size attribute of the instruction determines whether the stack pointer is incremented by2 or 4: this means that segment register pops in BITS 32 mode will pop 4 bytes off the stack anddiscard the upper two of them. If you need to override that, you can use an o16 or o32 prefix.

The above opcode listings give two forms for general−purpose register pop instructions: forexample, POP BX has the two forms 5B and 8F C3 . NASM will always generate the shorter formwhen given POP BX. NDISASM will disassemble both.

POP CS is not a documented instruction, and is not supported on any processor above the 8086(since they use 0Fh as an opcode prefix for instruction set extensions). However, at least some8086 processors do support it, and so NASM generates it for completeness.

B.4.245 POPAx: Pop All General−Purpose RegistersPOPA ; 61 [186] POPAW ; o16 61 [186] POPAD ; o32 61 [386]

• POPAW pops a word from the stack into each of, successively, DI , SI , BP, nothing (it discards aword from the stack which was a placeholder for SP), BX, DX, CX and AX. It is intended toreverse the operation of PUSHAW (see section B.4.264), but it ignores the value for SP that waspushed on the stack by PUSHAW.

• POPAD pops twice as much data, and places the results in EDI , ESI , EBP, nothing (placeholderfor ESP), EBX, EDX, ECX and EAX. It reverses the operation of PUSHAD.

POPA is an alias mnemonic for either POPAW or POPAD, depending on the current BITS setting.

Note that the registers are popped in reverse order of their numeric values in opcodes (see sectionB.2.1).

B.4.246 POPFx: Pop Flags RegisterPOPF ; 9D [8086] POPFW ; o16 9D [8086] POPFD ; o32 9D [386]

• POPFW pops a word from the stack and stores it in the bottom 16 bits of the flags register (or thewhole flags register, on processors below a 386).

• POPFD pops a doubleword and stores it in the entire flags register.

POPF is an alias mnemonic for either POPFW or POPFD, depending on the current BITS setting.

See also PUSHF (section B.4.265).

B.4.247 POR: MMX Bitwise ORPOR mm1,mm2/m64 ; 0F EB /r [PENT,MMX] POR xmm1,xmm2/m128 ; 66 0F EB /r [WILLAMETTE,SSE2]

POR performs a bitwise OR operation between its two operands (i.e. each bit of the result is 1 if andonly if at least one of the corresponding bits of the two inputs was 1), and stores the result in thedestination (first) operand.

B.4.248 PREFETCH: Prefetch Data Into CachesPREFETCH mem8 ; 0F 0D /0 [PENT,3DNOW] PREFETCHW mem8 ; 0F 0D /1 [PENT,3DNOW]

PREFETCH and PREFETCHW fetch the line of data from memory that contains the specified byte.PREFETCHW performs differently on the Athlon to earlier processors.

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For more details, see the 3DNow! Technology Manual.

B.4.249 PREFETCHh: Prefetch Data Into Caches PREFETCHNTA m8 ; 0F 18 /0 [KATMAI] PREFETCHT0 m8 ; 0F 18 /1 [KATMAI] PREFETCHT1 m8 ; 0F 18 /2 [KATMAI] PREFETCHT2 m8 ; 0F 18 /3 [KATMAI]

The PREFETCHh instructions fetch the line of data from memory that contains the specified byte. Itis placed in the cache according to rules specified by locality hints h:

The hints are:

• T0 (temporal data) – prefetch data into all levels of the cache hierarchy.

• T1 (temporal data with respect to first level cache) – prefetch data into level 2 cache and higher.

• T2 (temporal data with respect to second level cache) – prefetch data into level 2 cache andhigher.

• NTA (non−temporal data with respect to all cache levels) – prefetch data into non−temporalcache structure and into a location close to the processor, minimizing cache pollution.

Note that this group of instructions doesn’t provide a guarantee that the data will be in the cachewhen it is needed. For more details, see the Intel IA32 Software Developer Manual, Volume 2.

B.4.250 PSADBW: Packed Sum of Absolute DifferencesPSADBW mm1,mm2/m64 ; 0F F6 /r [KATMAI,MMX] PSADBW xmm1,xmm2/m128 ; 66 0F F6 /r [WILLAMETTE,SSE2]

PSADBW The PSADBW instruction computes the absolute value of the difference of the packedunsigned bytes in the two source operands. These differences are then summed to produce a wordresult in the lower 16−bit field of the destination register; the rest of the register is cleared. Thedestination operand is an MMX or an XMM register. The source operand can either be a register or amemory operand.

B.4.251 PSHUFD: Shuffle Packed DoublewordsPSHUFD xmm1,xmm2/m128,imm8 ; 66 0F 70 /r ib [WILLAMETTE,SSE2]

PSHUFD shuffles the doublewords in the source (second) operand according to the encodingspecified by imm8, and stores the result in the destination (first) operand.

Bits 0 and 1 of imm8 encode the source position of the doubleword to be copied to position 0 in thedestination operand. Bits 2 and 3 encode for position 1, bits 4 and 5 encode for position 2, and bits 6and 7 encode for position 3. For example, an encoding of 10 in bits 0 and 1 of imm8 indicates thatthe doubleword at bits 64−95 of the source operand will be copied to bits 0−31 of the destination.

B.4.252 PSHUFHW: Shuffle Packed High WordsPSHUFHW xmm1,xmm2/m128,imm8 ; F3 0F 70 /r ib [WILLAMETTE,SSE2]

PSHUFW shuffles the words in the high quadword of the source (second) operand according to theencoding specified by imm8, and stores the result in the high quadword of the destination (first)operand.

The operation of this instruction is similar to the PSHUFW instruction, except that the source anddestination are the top quadword of a 128−bit operand, instead of being 64−bit operands. The lowquadword is copied from the source to the destination without any changes.

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B.4.253 PSHUFLW: Shuffle Packed Low WordsPSHUFLW xmm1,xmm2/m128,imm8 ; F2 0F 70 /r ib [WILLAMETTE,SSE2]

PSHUFLW shuffles the words in the low quadword of the source (second) operand according to theencoding specified by imm8, and stores the result in the low quadword of the destination (first)operand.

The operation of this instruction is similar to the PSHUFW instruction, except that the source anddestination are the low quadword of a 128−bit operand, instead of being 64−bit operands. The highquadword is copied from the source to the destination without any changes.

B.4.254 PSHUFW: Shuffle Packed WordsPSHUFW mm1,mm2/m64,imm8 ; 0F 70 /r ib [KATMAI,MMX]

PSHUFW shuffles the words in the source (second) operand according to the encoding specified byimm8, and stores the result in the destination (first) operand.

Bits 0 and 1 of imm8 encode the source position of the word to be copied to position 0 in thedestination operand. Bits 2 and 3 encode for position 1, bits 4 and 5 encode for position 2, and bits 6and 7 encode for position 3. For example, an encoding of 10 in bits 0 and 1 of imm8 indicates thatthe word at bits 32−47 of the source operand will be copied to bits 0−15 of the destination.

B.4.255 PSLLx: Packed Data Bit Shift Left LogicalPSLLW mm1,mm2/m64 ; 0F F1 /r [PENT,MMX] PSLLW mm,imm8 ; 0F 71 /6 ib [PENT,MMX]

PSLLW xmm1,xmm2/m128 ; 66 0F F1 /r [WILLAMETTE,SSE2] PSLLW xmm,imm8 ; 66 0F 71 /6 ib [WILLAMETTE,SSE2]

PSLLD mm1,mm2/m64 ; 0F F2 /r [PENT,MMX] PSLLD mm,imm8 ; 0F 72 /6 ib [PENT,MMX]

PSLLD xmm1,xmm2/m128 ; 66 0F F2 /r [WILLAMETTE,SSE2] PSLLD xmm,imm8 ; 66 0F 72 /6 ib [WILLAMETTE,SSE2]

PSLLQ mm1,mm2/m64 ; 0F F3 /r [PENT,MMX] PSLLQ mm,imm8 ; 0F 73 /6 ib [PENT,MMX]

PSLLQ xmm1,xmm2/m128 ; 66 0F F3 /r [WILLAMETTE,SSE2] PSLLQ xmm,imm8 ; 66 0F 73 /6 ib [WILLAMETTE,SSE2]

PSLLDQ xmm1,imm8 ; 66 0F 73 /7 ib [PENT,MMX]

PSLLx performs logical left shifts of the data elements in the destination (first) operand, movingeach bit in the separate elements left by the number of bits specified in the source (second) operand,clearing the low−order bits as they are vacated.

• PSLLW shifts word sized elements.

• PSLLD shifts doubleword sized elements.

• PSLLQ shifts quadword sized elements.

• PSLLDQ shifts double quadword sized elements.

B.4.256 PSRAx: Packed Data Bit Shift Right ArithmeticPSRAW mm1,mm2/m64 ; 0F E1 /r [PENT,MMX] PSRAW mm,imm8 ; 0F 71 /4 ib [PENT,MMX]

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PSRAW xmm1,xmm2/m128 ; 66 0F E1 /r [WILLAMETTE,SSE2] PSRAW xmm,imm8 ; 66 0F 71 /4 ib [WILLAMETTE,SSE2]

PSRAD mm1,mm2/m64 ; 0F E2 /r [PENT,MMX] PSRAD mm,imm8 ; 0F 72 /4 ib [PENT,MMX]

PSRAD xmm1,xmm2/m128 ; 66 0F E2 /r [WILLAMETTE,SSE2] PSRAD xmm,imm8 ; 66 0F 72 /4 ib [WILLAMETTE,SSE2]

PSRAx performs arithmetic right shifts of the data elements in the destination (first) operand,moving each bit in the separate elements right by the number of bits specified in the source (second)operand, setting the high−order bits to the value of the original sign bit.

• PSRAW shifts word sized elements.

• PSRAD shifts doubleword sized elements.

B.4.257 PSRLx: Packed Data Bit Shift Right LogicalPSRLW mm1,mm2/m64 ; 0F D1 /r [PENT,MMX] PSRLW mm,imm8 ; 0F 71 /2 ib [PENT,MMX]

PSRLW xmm1,xmm2/m128 ; 66 0F D1 /r [WILLAMETTE,SSE2] PSRLW xmm,imm8 ; 66 0F 71 /2 ib [WILLAMETTE,SSE2]

PSRLD mm1,mm2/m64 ; 0F D2 /r [PENT,MMX] PSRLD mm,imm8 ; 0F 72 /2 ib [PENT,MMX]

PSRLD xmm1,xmm2/m128 ; 66 0F D2 /r [WILLAMETTE,SSE2] PSRLD xmm,imm8 ; 66 0F 72 /2 ib [WILLAMETTE,SSE2]

PSRLQ mm1,mm2/m64 ; 0F D3 /r [PENT,MMX] PSRLQ mm,imm8 ; 0F 73 /2 ib [PENT,MMX]

PSRLQ xmm1,xmm2/m128 ; 66 0F D3 /r [WILLAMETTE,SSE2] PSRLQ xmm,imm8 ; 66 0F 73 /2 ib [WILLAMETTE,SSE2]

PSRLDQ xmm1,imm8 ; 66 0F 73 /3 ib [WILLAMETTE,SSE2]

PSRLx performs logical right shifts of the data elements in the destination (first) operand, movingeach bit in the separate elements right by the number of bits specified in the source (second)operand, clearing the high−order bits as they are vacated.

• PSRLW shifts word sized elements.

• PSRLD shifts doubleword sized elements.

• PSRLQ shifts quadword sized elements.

• PSRLDQ shifts double quadword sized elements.

B.4.258 PSUBx: Subtract Packed IntegersPSUBB mm1,mm2/m64 ; 0F F8 /r [PENT,MMX] PSUBW mm1,mm2/m64 ; 0F F9 /r [PENT,MMX] PSUBD mm1,mm2/m64 ; 0F FA /r [PENT,MMX] PSUBQ mm1,mm2/m64 ; 0F FB /r [WILLAMETTE,SSE2]

PSUBB xmm1,xmm2/m128 ; 66 0F F8 /r [WILLAMETTE,SSE2] PSUBW xmm1,xmm2/m128 ; 66 0F F9 /r [WILLAMETTE,SSE2] PSUBD xmm1,xmm2/m128 ; 66 0F FA /r [WILLAMETTE,SSE2] PSUBQ xmm1,xmm2/m128 ; 66 0F FB /r [WILLAMETTE,SSE2]

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PSUBx subtracts packed integers in the source operand from those in the destination operand. Itdoesn’t differentiate between signed and unsigned integers, and doesn’t set any of the flags.

• PSUBB operates on byte sized elements.

• PSUBW operates on word sized elements.

• PSUBD operates on doubleword sized elements.

• PSUBQ operates on quadword sized elements.

B.4.259 PSUBSxx, PSUBUSx: Subtract Packed Integers With SaturationPSUBSB mm1,mm2/m64 ; 0F E8 /r [PENT,MMX] PSUBSW mm1,mm2/m64 ; 0F E9 /r [PENT,MMX]

PSUBSB xmm1,xmm2/m128 ; 66 0F E8 /r [WILLAMETTE,SSE2] PSUBSW xmm1,xmm2/m128 ; 66 0F E9 /r [WILLAMETTE,SSE2]

PSUBUSB mm1,mm2/m64 ; 0F D8 /r [PENT,MMX] PSUBUSW mm1,mm2/m64 ; 0F D9 /r [PENT,MMX]

PSUBUSB xmm1,xmm2/m128 ; 66 0F D8 /r [WILLAMETTE,SSE2] PSUBUSW xmm1,xmm2/m128 ; 66 0F D9 /r [WILLAMETTE,SSE2]

PSUBSx and PSUBUSx subtracts packed integers in the source operand from those in thedestination operand, and use saturation for results that are outside the range supported by thedestination operand.

• PSUBSB operates on signed bytes, and uses signed saturation on the results.

• PSUBSW operates on signed words, and uses signed saturation on the results.

• PSUBUSB operates on unsigned bytes, and uses signed saturation on the results.

• PSUBUSW operates on unsigned words, and uses signed saturation on the results.

B.4.260 PSUBSIW: MMX Packed Subtract with Saturation to Implied DestinationPSUBSIW mm1,mm2/m64 ; 0F 55 /r [CYRIX,MMX]

PSUBSIW, specific to the Cyrix extensions to the MMX instruction set, performs the same functionas PSUBSW, except that the result is not placed in the register specified by the first operand, butinstead in the implied destination register, specified as for PADDSIW (section B.4.200).

B.4.261 PSWAPD: Swap Packed Data PSWAPD mm1,mm2/m64 ; 0F 0F /r BB [PENT,3DNOW]

PSWAPD swaps the packed doublewords in the source operand, and stores the result in thedestination operand.

In the K6−2 and K6−III processors, this opcode uses the mnemonic PSWAPW, and it swaps theorder of words when copying from the source to the destination.

The operation in the K6−2 and K6−III processors is

dst[0−15] = src[48−63]; dst[16−31] = src[32−47]; dst[32−47] = src[16−31]; dst[48−63] = src[0−15].

The operation in the K6−x+ , ATHLON and later processors is:

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dst[0−31] = src[32−63]; dst[32−63] = src[0−31].

B.4.262 PUNPCKxxx: Unpack and Interleave DataPUNPCKHBW mm1,mm2/m64 ; 0F 68 /r [PENT,MMX] PUNPCKHWD mm1,mm2/m64 ; 0F 69 /r [PENT,MMX] PUNPCKHDQ mm1,mm2/m64 ; 0F 6A /r [PENT,MMX]

PUNPCKHBW xmm1,xmm2/m128 ; 66 0F 68 /r [WILLAMETTE,SSE2] PUNPCKHWD xmm1,xmm2/m128 ; 66 0F 69 /r [WILLAMETTE,SSE2] PUNPCKHDQ xmm1,xmm2/m128 ; 66 0F 6A /r [WILLAMETTE,SSE2] PUNPCKHQDQ xmm1,xmm2/m128 ; 66 0F 6D /r [WILLAMETTE,SSE2]

PUNPCKLBW mm1,mm2/m32 ; 0F 60 /r [PENT,MMX] PUNPCKLWD mm1,mm2/m32 ; 0F 61 /r [PENT,MMX] PUNPCKLDQ mm1,mm2/m32 ; 0F 62 /r [PENT,MMX]

PUNPCKLBW xmm1,xmm2/m128 ; 66 0F 60 /r [WILLAMETTE,SSE2] PUNPCKLWD xmm1,xmm2/m128 ; 66 0F 61 /r [WILLAMETTE,SSE2] PUNPCKLDQ xmm1,xmm2/m128 ; 66 0F 62 /r [WILLAMETTE,SSE2] PUNPCKLQDQ xmm1,xmm2/m128 ; 66 0F 6C /r [WILLAMETTE,SSE2]

PUNPCKxx all treat their operands as vectors, and produce a new vector generated by interleavingelements from the two inputs. The PUNPCKHxx instructions start by throwing away the bottom halfof each input operand, and the PUNPCKLxx instructions throw away the top half.

The remaining elements, are then interleaved into the destination, alternating elements from thesecond (source) operand and the first (destination) operand: so the leftmost part of each element inthe result always comes from the second operand, and the rightmost from the destination.

• PUNPCKxBW works a byte at a time, producing word sized output elements.

• PUNPCKxWD works a word at a time, producing doubleword sized output elements.

• PUNPCKxDQ works a doubleword at a time, producing quadword sized output elements.

• PUNPCKxQDQ works a quadword at a time, producing double quadword sized output elements.

So, for example, for MMX operands, if the first operand held 0x7A6A5A4A3A2A1A0A and thesecond held 0x7B6B5B4B3B2B1B0B, then:

• PUNPCKHBW would return 0x7B7A6B6A5B5A4B4A.

• PUNPCKHWD would return 0x7B6B7A6A5B4B5A4A.

• PUNPCKHDQ would return 0x7B6B5B4B7A6A5A4A.

• PUNPCKLBW would return 0x3B3A2B2A1B1A0B0A.

• PUNPCKLWD would return 0x3B2B3A2A1B0B1A0A.

• PUNPCKLDQ would return 0x3B2B1B0B3A2A1A0A.

B.4.263 PUSH: Push Data on StackPUSH reg16 ; o16 50+r [8086] PUSH reg32 ; o32 50+r [386]

PUSH r/m16 ; o16 FF /6 [8086] PUSH r/m32 ; o32 FF /6 [386]

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PUSH CS ; 0E [8086] PUSH DS ; 1E [8086] PUSH ES ; 06 [8086] PUSH SS ; 16 [8086] PUSH FS ; 0F A0 [386] PUSH GS ; 0F A8 [386]

PUSH imm8 ; 6A ib [186] PUSH imm16 ; o16 68 iw [186] PUSH imm32 ; o32 68 id [386]

PUSH decrements the stack pointer (SP or ESP) by 2 or 4, and then stores the given value at[SS:SP] or [SS:ESP] .

The address−size attribute of the instruction determines whether SP or ESP is used as the stackpointer: to deliberately override the default given by the BITS setting, you can use an a16 or a32prefix.

The operand−size attribute of the instruction determines whether the stack pointer is decrementedby 2 or 4: this means that segment register pushes in BITS 32 mode will push 4 bytes on the stack,of which the upper two are undefined. If you need to override that, you can use an o16 or o32prefix.

The above opcode listings give two forms for general−purpose register push instructions: forexample, PUSH BX has the two forms 53 and FF F3 . NASM will always generate the shorterform when given PUSH BX. NDISASM will disassemble both.

Unlike the undocumented and barely supported POP CS, PUSH CS is a perfectly valid andsensible instruction, supported on all processors.

The instruction PUSH SP may be used to distinguish an 8086 from later processors: on an 8086,the value of SP stored is the value it has after the push instruction, whereas on later processors it isthe value before the push instruction.

B.4.264 PUSHAx: Push All General−Purpose RegistersPUSHA ; 60 [186] PUSHAD ; o32 60 [386] PUSHAW ; o16 60 [186]

PUSHAW pushes, in succession, AX, CX, DX, BX, SP, BP, SI and DI on the stack, decrementing thestack pointer by a total of 16.

PUSHAD pushes, in succession, EAX, ECX, EDX, EBX, ESP, EBP, ESI and EDI on the stack,decrementing the stack pointer by a total of 32.

In both cases, the value of SP or ESP pushed is its original value, as it had before the instructionwas executed.

PUSHA is an alias mnemonic for either PUSHAW or PUSHAD, depending on the current BITSsetting.

Note that the registers are pushed in order of their numeric values in opcodes (see section B.2.1).

See also POPA (section B.4.245).

B.4.265 PUSHFx: Push Flags RegisterPUSHF ; 9C [8086] PUSHFD ; o32 9C [386] PUSHFW ; o16 9C [8086]

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• PUSHFW pops a word from the stack and stores it in the bottom 16 bits of the flags register (orthe whole flags register, on processors below a 386).

• PUSHFD pops a doubleword and stores it in the entire flags register.

PUSHF is an alias mnemonic for either PUSHFW or PUSHFD, depending on the current BITSsetting.

See also POPF (section B.4.246).

B.4.266 PXOR: MMX Bitwise XORPXOR mm1,mm2/m64 ; 0F EF /r [PENT,MMX] PXOR xmm1,xmm2/m128 ; 66 0F EF /r [WILLAMETTE,SSE2]

PXOR performs a bitwise XOR operation between its two operands (i.e. each bit of the result is 1 ifand only if exactly one of the corresponding bits of the two inputs was 1), and stores the result inthe destination (first) operand.

B.4.267 RCL, RCR: Bitwise Rotate through Carry BitRCL r/m8,1 ; D0 /2 [8086] RCL r/m8,CL ; D2 /2 [8086] RCL r/m8,imm8 ; C0 /2 ib [186] RCL r/m16,1 ; o16 D1 /2 [8086] RCL r/m16,CL ; o16 D3 /2 [8086] RCL r/m16,imm8 ; o16 C1 /2 ib [186] RCL r/m32,1 ; o32 D1 /2 [386] RCL r/m32,CL ; o32 D3 /2 [386] RCL r/m32,imm8 ; o32 C1 /2 ib [386]

RCR r/m8,1 ; D0 /3 [8086] RCR r/m8,CL ; D2 /3 [8086] RCR r/m8,imm8 ; C0 /3 ib [186] RCR r/m16,1 ; o16 D1 /3 [8086] RCR r/m16,CL ; o16 D3 /3 [8086] RCR r/m16,imm8 ; o16 C1 /3 ib [186] RCR r/m32,1 ; o32 D1 /3 [386] RCR r/m32,CL ; o32 D3 /3 [386] RCR r/m32,imm8 ; o32 C1 /3 ib [386]

RCL and RCR perform a 9−bit, 17−bit or 33−bit bitwise rotation operation, involving the givensource/destination (first) operand and the carry bit. Thus, for example, in the operation RCL AL,1 ,a 9−bit rotation is performed in which AL is shifted left by 1, the top bit of AL moves into the carryflag, and the original value of the carry flag is placed in the low bit of AL.

The number of bits to rotate by is given by the second operand. Only the bottom five bits of therotation count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a C1 byte) form of RCL foo,1 byusing a BYTE prefix: RCL foo,BYTE 1 . Similarly with RCR.

B.4.268 RCPPS: Packed Single−Precision FP ReciprocalRCPPS xmm1,xmm2/m128 ; 0F 53 /r [KATMAI,SSE]

RCPPS returns an approximation of the reciprocal of the packed single−precision FP values fromxmm2/m128. The maximum error for this approximation is: |Error| <= 1.5 x 2^−12

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B.4.269 RCPSS: Scalar Single−Precision FP ReciprocalRCPSS xmm1,xmm2/m128 ; F3 0F 53 /r [KATMAI,SSE]

RCPSS returns an approximation of the reciprocal of the lower single−precision FP value fromxmm2/m32; the upper three fields are passed through from xmm1. The maximum error for thisapproximation is: |Error| <= 1.5 x 2^−12

B.4.270 RDMSR: Read Model−Specific RegistersRDMSR ; 0F 32 [PENT,PRIV]

RDMSR reads the processor Model−Specific Register (MSR) whose index is stored in ECX, andstores the result in EDX:EAX. See also WRMSR (section B.4.329).

B.4.271 RDPMC: Read Performance−Monitoring CountersRDPMC ; 0F 33 [P6]

RDPMC reads the processor performance−monitoring counter whose index is stored in ECX, andstores the result in EDX:EAX.

This instruction is available on P6 and later processors and on MMX class processors.

B.4.272 RDSHR: Read SMM Header Pointer RegisterRDSHR r/m32 ; 0F 36 /0 [386,CYRIX,SMM]

RDSHR reads the contents of the SMM header pointer register and saves it to the destinationoperand, which can be either a 32 bit memory location or a 32 bit register.

See also WRSHR (section B.4.330).

B.4.273 RDTSC: Read Time−Stamp CounterRDTSC ; 0F 31 [PENT]

RDTSC reads the processor’s time−stamp counter into EDX:EAX.

B.4.274 RET, RETF, RETN: Return from Procedure CallRET ; C3 [8086] RET imm16 ; C2 iw [8086]

RETF ; CB [8086] RETF imm16 ; CA iw [8086]

RETN ; C3 [8086] RETN imm16 ; C2 iw [8086]

• RET, and its exact synonym RETN, pop IP or EIP from the stack and transfer control to the newaddress. Optionally, if a numeric second operand is provided, they increment the stack pointer bya further imm16 bytes after popping the return address.

• RETF executes a far return: after popping IP /EIP , it then pops CS, and then increments thestack pointer by the optional argument if present.

B.4.275 ROL, ROR: Bitwise RotateROL r/m8,1 ; D0 /0 [8086] ROL r/m8,CL ; D2 /0 [8086] ROL r/m8,imm8 ; C0 /0 ib [186] ROL r/m16,1 ; o16 D1 /0 [8086] ROL r/m16,CL ; o16 D3 /0 [8086]

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ROL r/m16,imm8 ; o16 C1 /0 ib [186] ROL r/m32,1 ; o32 D1 /0 [386] ROL r/m32,CL ; o32 D3 /0 [386] ROL r/m32,imm8 ; o32 C1 /0 ib [386]

ROR r/m8,1 ; D0 /1 [8086] ROR r/m8,CL ; D2 /1 [8086] ROR r/m8,imm8 ; C0 /1 ib [186] ROR r/m16,1 ; o16 D1 /1 [8086] ROR r/m16,CL ; o16 D3 /1 [8086] ROR r/m16,imm8 ; o16 C1 /1 ib [186] ROR r/m32,1 ; o32 D1 /1 [386] ROR r/m32,CL ; o32 D3 /1 [386] ROR r/m32,imm8 ; o32 C1 /1 ib [386]

ROL and ROR perform a bitwise rotation operation on the given source/destination (first) operand.Thus, for example, in the operation ROL AL,1 , an 8−bit rotation is performed in which AL isshifted left by 1 and the original top bit of AL moves round into the low bit.

The number of bits to rotate by is given by the second operand. Only the bottom five bits of therotation count are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a C1 byte) form of ROL foo,1 byusing a BYTE prefix: ROL foo,BYTE 1 . Similarly with ROR.

B.4.276 RSDC: Restore Segment Register and DescriptorRSDC segreg,m80 ; 0F 79 /r [486,CYRIX,SMM]

RSDC restores a segment register (DS, ES, FS, GS, or SS) from mem80, and sets up its descriptor.

B.4.277 RSLDT: Restore Segment Register and DescriptorRSLDT m80 ; 0F 7B /0 [486,CYRIX,SMM]

RSLDT restores the Local Descriptor Table (LDTR) from mem80.

B.4.278 RSM: Resume from System−Management ModeRSM ; 0F AA [PENT]

RSM returns the processor to its normal operating mode when it was in System−Management Mode.

B.4.279 RSQRTPS: Packed Single−Precision FP Square Root ReciprocalRSQRTPS xmm1,xmm2/m128 ; 0F 52 /r [KATMAI,SSE]

RSQRTPS computes the approximate reciprocals of the square roots of the packed single−precisionfloating−point values in the source and stores the results in xmm1. The maximum error for thisapproximation is: |Error| <= 1.5 x 2^−12

B.4.280 RSQRTSS: Scalar Single−Precision FP Square Root ReciprocalRSQRTSS xmm1,xmm2/m128 ; F3 0F 52 /r [KATMAI,SSE]

RSQRTSS returns an approximation of the reciprocal of the square root of the lowest ordersingle−precision FP value from the source, and stores it in the low doubleword of the destinationregister. The upper three fields of xmm1 are preserved. The maximum error for this approximationis: |Error| <= 1.5 x 2^−12

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B.4.281 RSTS: Restore TSR and DescriptorRSTS m80 ; 0F 7D /0 [486,CYRIX,SMM]

RSTS restores Task State Register (TSR) from mem80.

B.4.282 SAHF: Store AH to FlagsSAHF ; 9E [8086]

SAHF sets the low byte of the flags word according to the contents of the AH register.

The operation of SAHF is:

AH −−> SF:ZF:0:AF:0:PF:1:CF

See also LAHF (section B.4.131).

B.4.283 SAL, SAR: Bitwise Arithmetic ShiftsSAL r/m8,1 ; D0 /4 [8086] SAL r/m8,CL ; D2 /4 [8086] SAL r/m8,imm8 ; C0 /4 ib [186] SAL r/m16,1 ; o16 D1 /4 [8086] SAL r/m16,CL ; o16 D3 /4 [8086] SAL r/m16,imm8 ; o16 C1 /4 ib [186] SAL r/m32,1 ; o32 D1 /4 [386] SAL r/m32,CL ; o32 D3 /4 [386] SAL r/m32,imm8 ; o32 C1 /4 ib [386]

SAR r/m8,1 ; D0 /7 [8086] SAR r/m8,CL ; D2 /7 [8086] SAR r/m8,imm8 ; C0 /7 ib [186] SAR r/m16,1 ; o16 D1 /7 [8086] SAR r/m16,CL ; o16 D3 /7 [8086] SAR r/m16,imm8 ; o16 C1 /7 ib [186] SAR r/m32,1 ; o32 D1 /7 [386] SAR r/m32,CL ; o32 D3 /7 [386] SAR r/m32,imm8 ; o32 C1 /7 ib [386]

SAL and SAR perform an arithmetic shift operation on the given source/destination (first) operand.The vacated bits are filled with zero for SAL, and with copies of the original high bit of the sourceoperand for SAR.

SAL is a synonym for SHL (see section B.4.290). NASM will assemble either one to the same code,but NDISASM will always disassemble that code as SHL.

The number of bits to shift by is given by the second operand. Only the bottom five bits of the shiftcount are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a C1 byte) form of SAL foo,1 byusing a BYTE prefix: SAL foo,BYTE 1 . Similarly with SAR.

B.4.284 SALC: Set AL from Carry FlagSALC ; D6 [8086,UNDOC]

SALC is an early undocumented instruction similar in concept to SETcc (section B.4.287). Itsfunction is to set AL to zero if the carry flag is clear, or to 0xFF if it is set.

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B.4.285 SBB: Subtract with BorrowSBB r/m8,reg8 ; 18 /r [8086] SBB r/m16,reg16 ; o16 19 /r [8086] SBB r/m32,reg32 ; o32 19 /r [386]

SBB reg8,r/m8 ; 1A /r [8086] SBB reg16,r/m16 ; o16 1B /r [8086] SBB reg32,r/m32 ; o32 1B /r [386]

SBB r/m8,imm8 ; 80 /3 ib [8086] SBB r/m16,imm16 ; o16 81 /3 iw [8086] SBB r/m32,imm32 ; o32 81 /3 id [386]

SBB r/m16,imm8 ; o16 83 /3 ib [8086] SBB r/m32,imm8 ; o32 83 /3 ib [386]

SBB AL,imm8 ; 1C ib [8086] SBB AX,imm16 ; o16 1D iw [8086] SBB EAX,imm32 ; o32 1D id [386]

SBB performs integer subtraction: it subtracts its second operand, plus the value of the carry flag,from its first, and leaves the result in its destination (first) operand. The flags are set according tothe result of the operation: in particular, the carry flag is affected and can be used by a subsequentSBB instruction.


To subtract one number from another without also subtracting the contents of the carry flag, useSUB (section B.4.305).

B.4.286 SCASB, SCASW, SCASD: Scan StringSCASB ; AE [8086] SCASW ; o16 AF [8086] SCASD ; o32 AF [386]

SCASB compares the byte in AL with the byte at [ES:DI] or [ES:EDI] , and sets the flagsaccordingly. It then increments or decrements (depending on the direction flag: increments if theflag is clear, decrements if it is set) DI (or EDI ).


Segment override prefixes have no effect for this instruction: the use of ES for the load from [DI]or [EDI] cannot be overridden.

SCASW and SCASD work in the same way, but they compare a word to AX or a doubleword to EAXinstead of a byte to AL, and increment or decrement the addressing registers by 2 or 4 instead of 1.

The REPE and REPNE prefixes (equivalently, REPZ and REPNZ) may be used to repeat theinstruction up to CX (or ECX – again, the address size chooses which) times until the first unequal orequal byte is found.

B.4.287 SETcc: Set Register from ConditionSETcc r/m8 ; 0F 90+cc /2 [386]

SETcc sets the given 8−bit operand to zero if its condition is not satisfied, and to 1 if it is.

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B.4.288 SFENCE: Store FenceSFENCE ; 0F AE /7 [KATMAI]

SFENCE performs a serialising operation on all writes to memory that were issued before theSFENCE instruction. This guarantees that all memory writes before the SFENCE instruction arevisible before any writes after the SFENCE instruction.

SFENCE is ordered respective to other SFENCE instruction, MFENCE, any memory write and anyother serialising instruction (such as CPUID).

Weakly ordered memory types can be used to achieve higher processor performance through suchtechniques as out−of−order issue, write−combining, and write−collapsing. The degree to which aconsumer of data recognizes or knows that the data is weakly ordered varies among applicationsand may be unknown to the producer of this data. The SFENCE instruction provides aperformance−efficient way of insuring store ordering between routines that produceweakly−ordered results and routines that consume this data.

SFENCE uses the following ModRM encoding:



See also LFENCE (section B.4.137) and MFENCE (section B.4.151).

B.4.289 SGDT, SIDT , SLDT: Store Descriptor Table PointersSGDT mem ; 0F 01 /0 [286,PRIV] SIDT mem ; 0F 01 /1 [286,PRIV] SLDT r/m16 ; 0F 00 /0 [286,PRIV]

SGDT and SIDT both take a 6−byte memory area as an operand: they store the contents of theGDTR (global descriptor table register) or IDTR (interrupt descriptor table register) into that area asa 32−bit linear address and a 16−bit size limit from that area (in that order). These are the onlyinstructions which directly use linear addresses, rather than segment/offset pairs.

SLDT stores the segment selector corresponding to the LDT (local descriptor table) into the givenoperand.

See also LGDT, LIDT and LLDT (section B.4.138).

B.4.290 SHL, SHR: Bitwise Logical ShiftsSHL r/m8,1 ; D0 /4 [8086] SHL r/m8,CL ; D2 /4 [8086] SHL r/m8,imm8 ; C0 /4 ib [186] SHL r/m16,1 ; o16 D1 /4 [8086] SHL r/m16,CL ; o16 D3 /4 [8086] SHL r/m16,imm8 ; o16 C1 /4 ib [186] SHL r/m32,1 ; o32 D1 /4 [386] SHL r/m32,CL ; o32 D3 /4 [386] SHL r/m32,imm8 ; o32 C1 /4 ib [386]

SHR r/m8,1 ; D0 /5 [8086] SHR r/m8,CL ; D2 /5 [8086] SHR r/m8,imm8 ; C0 /5 ib [186]

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SHR r/m16,1 ; o16 D1 /5 [8086] SHR r/m16,CL ; o16 D3 /5 [8086] SHR r/m16,imm8 ; o16 C1 /5 ib [186] SHR r/m32,1 ; o32 D1 /5 [386] SHR r/m32,CL ; o32 D3 /5 [386] SHR r/m32,imm8 ; o32 C1 /5 ib [386]

SHL and SHR perform a logical shift operation on the given source/destination (first) operand. Thevacated bits are filled with zero.

A synonym for SHL is SAL (see section B.4.283). NASM will assemble either one to the samecode, but NDISASM will always disassemble that code as SHL.

The number of bits to shift by is given by the second operand. Only the bottom five bits of the shiftcount are considered by processors above the 8086.

You can force the longer (286 and upwards, beginning with a C1 byte) form of SHL foo,1 byusing a BYTE prefix: SHL foo,BYTE 1 . Similarly with SHR.

B.4.291 SHLD, SHRD: Bitwise Double−Precision ShiftsSHLD r/m16,reg16,imm8 ; o16 0F A4 /r ib [386] SHLD r/m16,reg32,imm8 ; o32 0F A4 /r ib [386] SHLD r/m16,reg16,CL ; o16 0F A5 /r [386] SHLD r/m16,reg32,CL ; o32 0F A5 /r [386]

SHRD r/m16,reg16,imm8 ; o16 0F AC /r ib [386] SHRD r/m32,reg32,imm8 ; o32 0F AC /r ib [386] SHRD r/m16,reg16,CL ; o16 0F AD /r [386] SHRD r/m32,reg32,CL ; o32 0F AD /r [386]

• SHLD performs a double−precision left shift. It notionally places its second operand to the rightof its first, then shifts the entire bit string thus generated to the left by a number of bits specifiedin the third operand. It then updates only the first operand according to the result of this. Thesecond operand is not modified.

• SHRD performs the corresponding right shift: it notionally places the second operand to the left ofthe first, shifts the whole bit string right, and updates only the first operand.

For example, if EAX holds 0x01234567 and EBX holds 0x89ABCDEF, then the instructionSHLD EAX,EBX,4 would update EAX to hold 0x12345678 . Under the same conditions,SHRD EAX,EBX,4 would update EAX to hold 0xF0123456 .

The number of bits to shift by is given by the third operand. Only the bottom five bits of the shiftcount are considered.

B.4.292 SHUFPD: Shuffle Packed Double−Precision FP ValuesSHUFPD xmm1,xmm2/m128,imm8 ; 66 0F C6 /r ib [WILLAMETTE,SSE2]

SHUFPD moves one of the packed double−precision FP values from the destination operand intothe low quadword of the destination operand; the upper quadword is generated by moving one ofthe double−precision FP values from the source operand into the destination. The select (third)operand selects which of the values are moved to the destination register.

The select operand is an 8−bit immediate: bit 0 selects which value is moved from the destinationoperand to the result (where 0 selects the low quadword and 1 selects the high quadword) and bit 1selects which value is moved from the source operand to the result. Bits 2 through 7 of the shuffleoperand are reserved.

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B.4.293 SHUFPS: Shuffle Packed Single−Precision FP ValuesSHUFPS xmm1,xmm2/m128,imm8 ; 0F C6 /r ib [KATMAI,SSE]

SHUFPS moves two of the packed single−precision FP values from the destination operand into thelow quadword of the destination operand; the upper quadword is generated by moving two of thesingle−precision FP values from the source operand into the destination. The select (third) operandselects which of the values are moved to the destination register.

The select operand is an 8−bit immediate: bits 0 and 1 select the value to be moved from thedestination operand the low doubleword of the result, bits 2 and 3 select the value to be moved fromthe destination operand the second doubleword of the result, bits 4 and 5 select the value to bemoved from the source operand the third doubleword of the result, and bits 6 and 7 select the valueto be moved from the source operand to the high doubleword of the result.

B.4.294 SMI: System Management InterruptSMI ; F1 [386,UNDOC]

SMI puts some AMD processors into SMM mode. It is available on some 386 and 486 processors,and is only available when DR7 bit 12 is set, otherwise it generates an Int 1.

B.4.295 SMINT, SMINTOLD: Software SMM Entry (CYRIX)SMINT ; 0F 38 [PENT,CYRIX] SMINTOLD ; 0F 7E [486,CYRIX]

SMINT puts the processor into SMM mode. The CPU state information is saved in the SMMmemory header, and then execution begins at the SMM base address.

SMINTOLD is the same as SMINT, but was the opcode used on the 486.

This pair of opcodes are specific to the Cyrix and compatible range of processors (Cyrix, IBM, Via).

B.4.296 SMSW: Store Machine Status WordSMSW r/m16 ; 0F 01 /4 [286,PRIV]

SMSW stores the bottom half of the CR0 control register (or the Machine Status Word, on 286processors) into the destination operand. See also LMSW (section B.4.139).

For 32−bit code, this would use the low 16−bits of the specified register (or a 16bit memorylocation), without needing an operand size override byte.

B.4.297 SQRTPD: Packed Double−Precision FP Square RootSQRTPD xmm1,xmm2/m128 ; 66 0F 51 /r [WILLAMETTE,SSE2]

SQRTPD calculates the square root of the packed double−precision FP value from the sourceoperand, and stores the double−precision results in the destination register.

B.4.298 SQRTPS: Packed Single−Precision FP Square RootSQRTPS xmm1,xmm2/m128 ; 0F 51 /r [KATMAI,SSE]

SQRTPS calculates the square root of the packed single−precision FP value from the sourceoperand, and stores the single−precision results in the destination register.

B.4.299 SQRTSD: Scalar Double−Precision FP Square RootSQRTSD xmm1,xmm2/m128 ; F2 0F 51 /r [WILLAMETTE,SSE2]

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SQRTSD calculates the square root of the low−order double−precision FP value from the sourceoperand, and stores the double−precision result in the destination register. The high−quadwordremains unchanged.

B.4.300 SQRTSS: Scalar Single−Precision FP Square RootSQRTSS xmm1,xmm2/m128 ; F3 0F 51 /r [KATMAI,SSE]

SQRTSS calculates the square root of the low−order single−precision FP value from the sourceoperand, and stores the single−precision result in the destination register. The three highdoublewords remain unchanged.

B.4.301 STC, STD, STI : Set FlagsSTC ; F9 [8086] STD ; FD [8086] STI ; FB [8086]

These instructions set various flags. STC sets the carry flag; STD sets the direction flag; and STIsets the interrupt flag (thus enabling interrupts).

To clear the carry, direction, or interrupt flags, use the CLC, CLD and CLI instructions (sectionB.4.20). To invert the carry flag, use CMC (section B.4.22).

B.4.302 STMXCSR: Store Streaming SIMD Extension Control/StatusSTMXCSR m32 ; 0F AE /3 [KATMAI,SSE]

STMXCSR stores the contents of the MXCSR control/status register to the specified memory location.MXCSR is used to enable masked/unmasked exception handling, to set rounding modes, to setflush−to−zero mode, and to view exception status flags. The reserved bits in the MXCSR register arestored as 0s.

For details of the MXCSR register, see the Intel processor docs.

See also LDMXCSR (section B.4.133).

B.4.303 STOSB, STOSW, STOSD: Store Byte to StringSTOSB ; AA [8086] STOSW ; o16 AB [8086] STOSD ; o32 AB [386]

STOSB stores the byte in AL at [ES:DI] or [ES:EDI] , and sets the flags accordingly. It thenincrements or decrements (depending on the direction flag: increments if the flag is clear,decrements if it is set) DI (or EDI ).


Segment override prefixes have no effect for this instruction: the use of ES for the store to [DI] or[EDI] cannot be overridden.

STOSW and STOSD work in the same way, but they store the word in AX or the doubleword in EAXinstead of the byte in AL, and increment or decrement the addressing registers by 2 or 4 instead of 1.


B.4.304 STR: Store Task RegisterSTR r/m16 ; 0F 00 /1 [286,PRIV]

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STR stores the segment selector corresponding to the contents of the Task Register into its operand.When the operand size is a 16−bit register, the upper 16−bits are cleared to 0s. When the destinationoperand is a memory location, 16 bits are written regardless of the operand size.

B.4.305 SUB: Subtract IntegersSUB r/m8,reg8 ; 28 /r [8086] SUB r/m16,reg16 ; o16 29 /r [8086] SUB r/m32,reg32 ; o32 29 /r [386]

SUB reg8,r/m8 ; 2A /r [8086] SUB reg16,r/m16 ; o16 2B /r [8086] SUB reg32,r/m32 ; o32 2B /r [386]

SUB r/m8,imm8 ; 80 /5 ib [8086] SUB r/m16,imm16 ; o16 81 /5 iw [8086] SUB r/m32,imm32 ; o32 81 /5 id [386]

SUB r/m16,imm8 ; o16 83 /5 ib [8086] SUB r/m32,imm8 ; o32 83 /5 ib [386]

SUB AL,imm8 ; 2C ib [8086] SUB AX,imm16 ; o16 2D iw [8086] SUB EAX,imm32 ; o32 2D id [386]

SUB performs integer subtraction: it subtracts its second operand from its first, and leaves the resultin its destination (first) operand. The flags are set according to the result of the operation: inparticular, the carry flag is affected and can be used by a subsequent SBB instruction (sectionB.4.285).


B.4.306 SUBPD: Packed Double−Precision FP SubtractSUBPD xmm1,xmm2/m128 ; 66 0F 5C /r [WILLAMETTE,SSE2]

SUBPD subtracts the packed double−precision FP values of the source operand from those of thedestination operand, and stores the result in the destination operation.

B.4.307 SUBPS: Packed Single−Precision FP SubtractSUBPS xmm1,xmm2/m128 ; 0F 5C /r [KATMAI,SSE]

SUBPS subtracts the packed single−precision FP values of the source operand from those of thedestination operand, and stores the result in the destination operation.

B.4.308 SUBSD: Scalar Single−FP SubtractSUBSD xmm1,xmm2/m128 ; F2 0F 5C /r [WILLAMETTE,SSE2]

SUBSD subtracts the low−order double−precision FP value of the source operand from that of thedestination operand, and stores the result in the destination operation. The high quadword isunchanged.

B.4.309 SUBSS: Scalar Single−FP SubtractSUBSS xmm1,xmm2/m128 ; F3 0F 5C /r [KATMAI,SSE]

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SUBSS subtracts the low−order single−precision FP value of the source operand from that of thedestination operand, and stores the result in the destination operation. The three high doublewordsare unchanged.

B.4.310 SVDC: Save Segment Register and DescriptorSVDC m80,segreg ; 0F 78 /r [486,CYRIX,SMM]

SVDC saves a segment register (DS, ES, FS, GS, or SS) and its descriptor to mem80.

B.4.311 SVLDT: Save LDTR and DescriptorSVLDT m80 ; 0F 7A /0 [486,CYRIX,SMM]

SVLDT saves the Local Descriptor Table (LDTR) to mem80.

B.4.312 SVTS: Save TSR and DescriptorSVTS m80 ; 0F 7C /0 [486,CYRIX,SMM]

SVTS saves the Task State Register (TSR) to mem80.

B.4.313 SYSCALL: Call Operating SystemSYSCALL ; 0F 05 [P6,AMD]

SYSCALL provides a fast method of transferring control to a fixed entry point in an operatingsystem.

• The EIP register is copied into the ECX register.

• Bits [31−0] of the 64−bit SYSCALL/SYSRET Target Address Register (STAR) are copied intothe EIP register.

• Bits [47−32] of the STAR register specify the selector that is copied into the CS register.

• Bits [47−32]+1000b of the STAR register specify the selector that is copied into the SS register.

The CS and SS registers should not be modified by the operating system between the execution ofthe SYSCALL instruction and its corresponding SYSRET instruction.

For more information, see the SYSCALL and SYSRET Instruction Specification(AMD document number 21086.pdf).

B.4.314 SYSENTER: Fast System CallSYSENTER ; 0F 34 [P6]

SYSENTER executes a fast call to a level 0 system procedure or routine. Before using thisinstruction, various MSRs need to be set up:

• SYSENTER_CS_MSR contains the 32−bit segment selector for the privilege level 0 codesegment. (This value is also used to compute the segment selector of the privilege level 0 stacksegment.)

• SYSENTER_EIP_MSR contains the 32−bit offset into the privilege level 0 code segment to thefirst instruction of the selected operating procedure or routine.

• SYSENTER_ESP_MSR contains the 32−bit stack pointer for the privilege level 0 stack.

SYSENTER performs the following sequence of operations:

• Loads the segment selector from the SYSENTER_CS_MSR into the CS register.

• Loads the instruction pointer from the SYSENTER_EIP_MSR into the EIP register.

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• Adds 8 to the value in SYSENTER_CS_MSR and loads it into the SS register.

• Loads the stack pointer from the SYSENTER_ESP_MSR into the ESP register.

• Switches to privilege level 0.

• Clears the VM flag in the EFLAGS register, if the flag is set.

• Begins executing the selected system procedure.

In particular, note that this instruction des not save the values of CS or (E)IP . If you need to returnto the calling code, you need to write your code to cater for this.

For more information, see the Intel Architecture Software Developer’s Manual, Volume 2.

B.4.315 SYSEXIT: Fast Return From System CallSYSEXIT ; 0F 35 [P6,PRIV]

SYSEXIT executes a fast return to privilege level 3 user code. This instruction is a companioninstruction to the SYSENTER instruction, and can only be executed by privilege level 0 code.Various registers need to be set up before calling this instruction:

• SYSENTER_CS_MSR contains the 32−bit segment selector for the privilege level 0 codesegment in which the processor is currently executing. (This value is used to compute thesegment selectors for the privilege level 3 code and stack segments.)

• EDX contains the 32−bit offset into the privilege level 3 code segment to the first instruction to beexecuted in the user code.

• ECX contains the 32−bit stack pointer for the privilege level 3 stack.

SYSEXIT performs the following sequence of operations:

• Adds 16 to the value in SYSENTER_CS_MSR and loads the sum into the CS selector register.

• Loads the instruction pointer from the EDX register into the EIP register.

• Adds 24 to the value in SYSENTER_CS_MSR and loads the sum into the SS selector register.

• Loads the stack pointer from the ECX register into the ESP register.

• Switches to privilege level 3.

• Begins executing the user code at the EIP address.

For more information on the use of the SYSENTER and SYSEXIT instructions, see the IntelArchitecture Software Developer’s Manual, Volume 2.

B.4.316 SYSRET: Return From Operating SystemSYSRET ; 0F 07 [P6,AMD,PRIV]

SYSRET is the return instruction used in conjunction with the SYSCALL instruction to provide fastentry/exit to an operating system.

• The ECX register, which points to the next sequential instruction after the correspondingSYSCALL instruction, is copied into the EIP register.

• Bits [63−48] of the STAR register specify the selector that is copied into the CS register.

• Bits [63−48]+1000b of the STAR register specify the selector that is copied into the SS register.

• Bits [1−0] of the SS register are set to 11b (RPL of 3) regardless of the value of bits [49−48] ofthe STAR register.

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The CS and SS registers should not be modified by the operating system between the execution ofthe SYSCALL instruction and its corresponding SYSRET instruction.

For more information, see the SYSCALL and SYSRET Instruction Specification(AMD document number 21086.pdf).

B.4.317 TEST: Test Bits (notional bitwise AND)TEST r/m8,reg8 ; 84 /r [8086] TEST r/m16,reg16 ; o16 85 /r [8086] TEST r/m32,reg32 ; o32 85 /r [386]

TEST r/m8,imm8 ; F6 /0 ib [8086] TEST r/m16,imm16 ; o16 F7 /0 iw [8086] TEST r/m32,imm32 ; o32 F7 /0 id [386]

TEST AL,imm8 ; A8 ib [8086] TEST AX,imm16 ; o16 A9 iw [8086] TEST EAX,imm32 ; o32 A9 id [386]

TEST performs a ‘mental’ bitwise AND of its two operands, and affects the flags as if the operationhad taken place, but does not store the result of the operation anywhere.

B.4.318 UCOMISD: Unordered Scalar Double−Precision FP compare and set EFLAGSUCOMISD xmm1,xmm2/m128 ; 66 0F 2E /r [WILLAMETTE,SSE2]

UCOMISD compares the low−order double−precision FP numbers in the two operands, and sets theZF, PF and CF bits in the EFLAGS register. In addition, the OF, SF and AF bits in the EFLAGSregister are zeroed out. The unordered predicate (ZF, PF and CF all set) is returned if either sourceoperand is a NaN (qNaN or sNaN).

B.4.319 UCOMISS: Unordered Scalar Single−Precision FP compare and set EFLAGSUCOMISS xmm1,xmm2/m128 ; 0F 2E /r [KATMAI,SSE]

UCOMISS compares the low−order single−precision FP numbers in the two operands, and sets theZF, PF and CF bits in the EFLAGS register. In addition, the OF, SF and AF bits in the EFLAGSregister are zeroed out. The unordered predicate (ZF, PF and CF all set) is returned if either sourceoperand is a NaN (qNaN or sNaN).

B.4.320 UD0, UD1, UD2: Undefined InstructionUD0 ; 0F FF [186,UNDOC] UD1 ; 0F B9 [186,UNDOC] UD2 ; 0F 0B [186]

UDx can be used to generate an invalid opcode exception, for testing purposes.

UD0 is specifically documented by AMD as being reserved for this purpose.

UD1 is documented by Intel as being available for this purpose.

UD2 is specifically documented by Intel as being reserved for this purpose. Intel document this asthe preferred method of generating an invalid opcode exception.

All these opcodes can be used to generate invalid opcode exceptions on all currently availableprocessors.

193

B.4.321 UMOV: User Move DataUMOV r/m8,reg8 ; 0F 10 /r [386,UNDOC] UMOV r/m16,reg16 ; o16 0F 11 /r [386,UNDOC] UMOV r/m32,reg32 ; o32 0F 11 /r [386,UNDOC]

UMOV reg8,r/m8 ; 0F 12 /r [386,UNDOC] UMOV reg16,r/m16 ; o16 0F 13 /r [386,UNDOC] UMOV reg32,r/m32 ; o32 0F 13 /r [386,UNDOC]

This undocumented instruction is used by in−circuit emulators to access user memory (as opposedto host memory). It is used just like an ordinary memory/register or register/register MOVinstruction, but accesses user space.

This instruction is only available on some AMD and IBM 386 and 486 processors.

B.4.322 UNPCKHPD: Unpack and Interleave High Packed Double−Precision FP ValuesUNPCKHPD xmm1,xmm2/m128 ; 66 0F 15 /r [WILLAMETTE,SSE2]

UNPCKHPD performs an interleaved unpack of the high−order data elements of the source anddestination operands, saving the result in xmm1. It ignores the lower half of the sources.


dst[63−0] := dst[127−64]; dst[127−64] := src[127−64].

B.4.323 UNPCKHPS: Unpack and Interleave High Packed Single−Precision FP ValuesUNPCKHPS xmm1,xmm2/m128 ; 0F 15 /r [KATMAI,SSE]

UNPCKHPS performs an interleaved unpack of the high−order data elements of the source anddestination operands, saving the result in xmm1. It ignores the lower half of the sources.


dst[31−0] := dst[95−64]; dst[63−32] := src[95−64]; dst[95−64] := dst[127−96]; dst[127−96] := src[127−96].

B.4.324 UNPCKLPD: Unpack and Interleave Low Packed Double−Precision FP DataUNPCKLPD xmm1,xmm2/m128 ; 66 0F 14 /r [WILLAMETTE,SSE2]

UNPCKLPD performs an interleaved unpack of the low−order data elements of the source anddestination operands, saving the result in xmm1. It ignores the lower half of the sources.


dst[63−0] := dst[63−0]; dst[127−64] := src[63−0].

B.4.325 UNPCKLPS: Unpack and Interleave Low Packed Single−Precision FP DataUNPCKLPS xmm1,xmm2/m128 ; 0F 14 /r [KATMAI,SSE]

UNPCKLPS performs an interleaved unpack of the low−order data elements of the source anddestination operands, saving the result in xmm1. It ignores the lower half of the sources.


194

dst[31−0] := dst[31−0]; dst[63−32] := src[31−0]; dst[95−64] := dst[63−32]; dst[127−96] := src[63−32].

B.4.326 VERR, VERW: Verify Segment Readability/WritabilityVERR r/m16 ; 0F 00 /4 [286,PRIV]

VERW r/m16 ; 0F 00 /5 [286,PRIV]

• VERR sets the zero flag if the segment specified by the selector in its operand can be read from atthe current privilege level. Otherwise it is cleared.

• VERW sets the zero flag if the segment can be written.

B.4.327 WAIT: Wait for Floating−Point ProcessorWAIT ; 9B [8086] FWAIT ; 9B [8086]

WAIT, on 8086 systems with a separate 8087 FPU, waits for the FPU to have finished any operationit is engaged in before continuing main processor operations, so that (for example) an FPU store tomain memory can be guaranteed to have completed before the CPU tries to read the result back out.

On higher processors, WAIT is unnecessary for this purpose, and it has the alternative purpose ofensuring that any pending unmasked FPU exceptions have happened before execution continues.

B.4.328 WBINVD: Write Back and Invalidate CacheWBINVD ; 0F 09 [486]

WBINVD invalidates and empties the processor’s internal caches, and causes the processor toinstruct external caches to do the same. It writes the contents of the caches back to memory first, sono data is lost. To flush the caches quickly without bothering to write the data back first, use INVD(section B.4.125).

B.4.329 WRMSR: Write Model−Specific RegistersWRMSR ; 0F 30 [PENT]

WRMSR writes the value in EDX:EAX to the processor Model−Specific Register (MSR) whoseindex is stored in ECX. See also RDMSR (section B.4.270).

B.4.330 WRSHR: Write SMM Header Pointer RegisterWRSHR r/m32 ; 0F 37 /0 [386,CYRIX,SMM]

WRSHR loads the contents of either a 32−bit memory location or a 32−bit register into the SMMheader pointer register.

See also RDSHR (section B.4.272).

B.4.331 XADD: Exchange and AddXADD r/m8,reg8 ; 0F C0 /r [486] XADD r/m16,reg16 ; o16 0F C1 /r [486] XADD r/m32,reg32 ; o32 0F C1 /r [486]

XADD exchanges the values in its two operands, and then adds them together and writes the resultinto the destination (first) operand. This instruction can be used with a LOCK prefix formulti−processor synchronisation purposes.

195

B.4.332 XBTS: Extract Bit StringXBTS reg16,r/m16 ; o16 0F A6 /r [386,UNDOC] XBTS reg32,r/m32 ; o32 0F A6 /r [386,UNDOC]

The implied operation of this instruction is:

XBTS r/m16,reg16,AX,CL XBTS r/m32,reg32,EAX,CL

Writes a bit string from the source operand to the destination. CL indicates the number of bits to becopied, and (E)AX indicates the low order bit offset in the source. The bits are written to the loworder bits of the destination register. For example, if CL is set to 4 and AX (for 16−bit code) is set to5, bits 5−8 of src will be copied to bits 0−3 of dst . This instruction is very poorly documented,and I have been unable to find any official source of documentation on it.

XBTS is supported only on the early Intel 386s, and conflicts with the opcodes for CMPXCHG486(on early Intel 486s). NASM supports it only for completeness. Its counterpart is IBTS (see sectionB.4.116).

B.4.333 XCHG: ExchangeXCHG reg8,r/m8 ; 86 /r [8086] XCHG reg16,r/m8 ; o16 87 /r [8086] XCHG reg32,r/m32 ; o32 87 /r [386]

XCHG r/m8,reg8 ; 86 /r [8086] XCHG r/m16,reg16 ; o16 87 /r [8086] XCHG r/m32,reg32 ; o32 87 /r [386]

XCHG AX,reg16 ; o16 90+r [8086] XCHG EAX,reg32 ; o32 90+r [386] XCHG reg16,AX ; o16 90+r [8086] XCHG reg32,EAX ; o32 90+r [386]

XCHG exchanges the values in its two operands. It can be used with a LOCK prefix for purposes ofmulti−processor synchronisation.

XCHG AX,AX or XCHG EAX,EAX (depending on the BITS setting) generates the opcode 90h ,and so is a synonym for NOP (section B.4.190).

B.4.334 XLATB: Translate Byte in Lookup TableXLAT ; D7 [8086] XLATB ; D7 [8086]

XLATB adds the value in AL, treated as an unsigned byte, to BX or EBX, and loads the byte from theresulting address (in the segment specified by DS) back into AL.

The base register used is BX if the address size is 16 bits, and EBX if it is 32 bits. If you need to usean address size not equal to the current BITS setting, you can use an explicit a16 or a32 prefix.

The segment register used to load from [BX+AL] or [EBX+AL] can be overridden by using asegment register name as a prefix (for example, es xlatb ).

B.4.335 XOR: Bitwise Exclusive ORXOR r/m8,reg8 ; 30 /r [8086] XOR r/m16,reg16 ; o16 31 /r [8086] XOR r/m32,reg32 ; o32 31 /r [386]

196

XOR reg8,r/m8 ; 32 /r [8086] XOR reg16,r/m16 ; o16 33 /r [8086] XOR reg32,r/m32 ; o32 33 /r [386]

XOR r/m8,imm8 ; 80 /6 ib [8086] XOR r/m16,imm16 ; o16 81 /6 iw [8086] XOR r/m32,imm32 ; o32 81 /6 id [386]

XOR r/m16,imm8 ; o16 83 /6 ib [8086] XOR r/m32,imm8 ; o32 83 /6 ib [386]

XOR AL,imm8 ; 34 ib [8086] XOR AX,imm16 ; o16 35 iw [8086] XOR EAX,imm32 ; o32 35 id [386]

XOR performs a bitwise XOR operation between its two operands (i.e. each bit of the result is 1 ifand only if exactly one of the corresponding bits of the two inputs was 1), and stores the result inthe destination (first) operand.


The MMX instruction PXOR (see section B.4.266) performs the same operation on the 64−bit MMXregisters.

B.4.336 XORPD: Bitwise Logical XOR of Double−Precision FP ValuesXORPD xmm1,xmm2/m128 ; 66 0F 57 /r [WILLAMETTE,SSE2]

XORPD returns a bit−wise logical XOR between the source and destination operands, storing theresult in the destination operand.

B.4.337 XORPS: Bitwise Logical XOR of Single−Precision FP ValuesXORPS xmm1,xmm2/m128 ; 0F 57 /r [KATMAI,SSE]

XORPS returns a bit−wise logical XOR between the source and destination operands, storing theresult in the destination operand.

197

Index

!= operator 52$$ token 36, 76$

Here token 36prefix 31, 34, 79

% operator 36%! 63%$ and %$$ prefixes 55, 56%% operator 36, 46%+ 43%+1 and %−1 syntax 50%0 parameter count 47, 48& operator 36&& operator 52* operator 36+ modifier 47+ operator

binary 36unary 36

− operatorbinary 36unary 36

..@ symbol prefix 39, 46/ operator 36// operator 36< operator 52<< operator 36<= operator 52<> operator 52= operator 52== operator 52> operator 52>= operator 52>> operator 36? MASM syntax 32^ operator 36^^ operator 52| operator 36|| operator 52~ operator 36−a option 26, 107a16 101, 124, 147, 152, 159, 162, 173,

180, 185, 189, 196a32 101, 124, 147, 152, 159, 162, 173,

180, 185, 189, 196a86 20, 28, 29AAA 115

AAD 115AAM 115AAS 115ABSOLUTE 65, 72ADC 115ADD 116addition 36ADDPD 117ADDPS 117addressing, mixed−size 100address−size prefixes 31ADDSD 117ADDSS 117algebra 34ALIGN 60, 69, 71ALIGNB 60alignment

in bin sections 70in elf sections 76in obj sections 71in win32 sections 75of elf common variables 77

ALINK 81alink.sourceforge.net 81alloc 76alt.lang.asm 20, 21ambiguity 29AND 117ANDNPD 118ANDNPS 118ANDPD 118ANDPS 119a.out

BSD version 78Linux version 78

aout 23, 78aoutb 78, 96%arg 61arg 89, 95ARPL 119as86 20, 23, 78assembler directives 64assembly passes 38assembly−time options 26%assign 43ASSUME 29AT 60

198

Autoconf 22autoexec.bat 21auto−sync 107−b 106bin 23, 24, 69

multisection 70binary 34binary files 3216−bit mode, versus 32−bit mode 64bit shift 36BITS 64, 69bitwise AND 36bitwise OR 36bitwise XOR 36block IFs 56boot loader 69boot sector 103Borland

Pascal 90Win32 compilers 70

BOUND 119braces

after % sign 49around macro parameters 45

BSD 96BSF 119BSR 119.bss 76, 78, 79BSWAP 119BT 120BTC 120BTR 120BTS 120bugs 104bugtracker 104BYTE 103C calling convention 86, 93C symbol names 84CALL 120CALL FAR 37case sensitivity 28, 41, 42, 43, 45, 52, 72CBW 121CDQ 121changing sections 64character constant 32, 35circular references 41CLASS 71CLC 121CLD 121%clear 58CLFLUSH 121CLI 121CLTS 121

c16.mac 89, 92c32.mac 95CMC 122CMOVcc 122CMP 122CMPccPD 122CMPccPS 123CMPccSD 124CMPccSS 125CMPEQPD 122CMPEQPS 123CMPEQSD 124CMPEQSS 125CMPLEPD 122CMPLEPS 123CMPLESD 124CMPLESS 125CMPLTPD 122CMPLTPS 123CMPLTSD 124CMPLTSS 125CMPNEQPD 122CMPNEQPS 123CMPNEQSD 124CMPNEQSS 125CMPNLEPD 122CMPNLEPS 123CMPNLESD 124CMPNLESS 125CMPNLTPD 122CMPNLTPS 123CMPNLTSD 124CMPNLTSS 125CMPORDPD 122CMPORDPS 123CMPORDSD 124CMPORDSS 125CMPSB 124CMPSD 124CMPSW 124CMPUNORDPD 122CMPUNORDPS 123CMPUNORDSD 124CMPUNORDSS 125CMPXCHG 125CMPXCHG486 125CMPXCHG8B 126coff 23, 75colon 31.COM 69, 83COMISD 126COMISS 126command−line 23, 69

199

commas in macro parameters 47COMMON 67, 71

elf extensions to 77obj extensions to 74

Common Object File Format 75common variables 67

alignment in elf 77element size 74

comp.lang.asm.x86 20, 21comp.os.linux.announce 21comp.os.msdos.programmer 84concatenating macro parameters 49condition codes 111condition codes as macro parameters 50condition predicates 123, 124, 125conditional assembly 50conditional jump 148conditional jumps 103conditional−return macro 50configure 22constants 34context stack 55, 56context−local labels 55context−local single−line macros 56control registers 111counting macro parameters 48CPU 67CPUID 35, 127creating contexts 55critical expression 32, 33, 38, 44, 66CVTDQ2PD 127CVTDQ2PS 127CVTPD2DQ 127CVTPD2PI 128CVTPD2PS 128CVTPI2PD 128CVTPI2PS 128CVTPS2DQ 128CVTPS2PD 129CVTPS2PI 129CVTSD2SI 129CVTSD2SS 129CVTSI2SD 129CVTSI2SS 130CVTSS2SD 130CVTSS2SI 130CVTTPD2DQ 130CVTTPD2PI 130CVTTPS2DQ 131CVTTPS2PI 131CVTTSD2SI 131CVTTSS2SI 131CWD 121

CWDE 121−D option 25−d option 25DAA 131DAS 131.data 76, 78, 79_DATA 86data 77, 79data structure 88, 95DB 32, 35dbg 79DD 32, 35debug information 24debug registers 111debugging format 24DEC 132declaring structures 59default macro parameters 47default name 69default−WRT mechanism 74%define 25, 41defining sections 64design goals 28DevPac 32, 39disabling listing expansion 50DIV 132division 36DIVPD 132DIVPS 133DIVSD 133DIVSS 133DJGPP 75, 93djlink 81DLL symbols

exporting 73importing 72

DOS 21, 24, 25DOS archive 21DOS source archive 21DQ 32, 35.drectve 75DT 32, 35DUP 30, 33DW 32, 35DWORD 32−E option 24−e option 26, 108effective addresses 31, 33, 38, 113element size, in common variables 74ELF 23, 75

shared libraries 7616−bit code and 78

%elif 50, 52

200

%elifctx 51%elifdef 51%elifid 53%elifidn 52%elifidni 52%elifmacro 51%elifnctx 51%elifndef 51%elifnid 53%elifnidn 52%elifnidni 52%elifnmacro 51%elifnnum 53%elifnstr 53%elifnum 53%elifstr 53%else 50e−mail 21EMMS 133endproc 89, 95%endrep 54ENDSTRUC 59, 66ENTER 133environment 28EQU 32, 33, 38%error 53error messages 24, 25EVEN 60.EXE 70, 81EXE_begin 82EXE2BIN 84exebin.mac 82exec 76Executable and Linkable Format 75EXE_end 82EXE_stack 82%exitrep 54EXPORT 73export 79exporting symbols 67expressions 26, 36extension 23, 69EXTERN 66

obj extensions to 73−F option 24−f option 24, 69FABS 134FADD 134FADDP 134far call 29, 121far common variables 74far jump 149far pointer 37

FARCODE 89, 92FBLD 134FBSTP 134FCHS 135FCLEX 135FCMOVcc 135FCOM 136FCOMI 136FCOMIP 136FCOMP 136FCOMPP 136FCOS 136FDECSTP 136FDIV 137FDIVP 137FDIVR 137FDIVRP 137FEMMS 137FFREE 137FIADD 138FICOM 138FICOMP 138FIDIV 138FIDIVR 138FILD 138__FILE__ 58FIMUL 138FINCSTP 138FINIT 139FIST 138FISTP 138FISUB 139FLAT 71flat memory model 93flat−form binary 69FLD 139FLDCW 139FLDENV 140FLDxx 139floating−point 29, 31, 32, 35

constants 35registers 111

FMUL 140FMULP 140FNINIT 139FNOP 140format−specific directives 64forward references 38FPATAN 140FPREM 140FPREM1 140FPTAN 140frame pointer 86, 90, 93

201

FreeBSD 78, 96FreeLink 81FRNDINT 141FRSTOR 141FSAVE 141FSCALE 141FSETPM 141FSIN 141FSINCOS 141FSQRT 141FST 142FSTCW 142FSTENV 142FSTP 142FSTSW 142FSUB 142FSUBP 142FSUBR 142FSUBRP 142ftp.kernel.org 21ftp.simtel.net 81FTST 143FUCOMxx 143function 77, 79functions

C calling convention 86, 93Pascal calling convention 90

FXAM 144FXCH 144FxDISI 136FxENI 136F2XM1 134FXRSTOR 144FXSAVE 144FXTRACT 144FYL2X 145FYL2XP1 145−g option 24gas 20gcc 20general purpose register 109GLOBAL 67

aoutb extensions to 77elf extensions to 77rdf extensions to 79

global offset table 96_GLOBAL_OFFSET_TABLE_ 76..got 76GOT relocations 98GOT 76, 96..gotoff 76GOTOFF relocations 97..gotpc 76

GOTPC relocations 97graphics 32greedy macro parameters 46GROUP 72groups 37−h 106hex 34HLT 145hybrid syntaxes 29−I option 25−i option 25, 107%iassign 43ibiblio.org 21IBTS 145ICEBP 147%idefine 41IDIV 145IEND 60%if 50, 52%ifctx 51, 56%ifdef 51%ifid 52%ifidn 52%ifidni 52ifmacro 51%ifnctx 51%ifndef 51%ifnid 53%ifnidn 52%ifnidni 52%ifnmacro 51%ifnnum 53%ifnstr 53%ifnum 52%ifstr 52%imacro 44immediate operand 109IMPORT 72import library 72importing symbols 66IMUL 146IN 146INC 146INCBIN 32, 35%include 25, 54include search path 25including other files 54inefficient code 103infinite loop 36informational section 75INSB 147INSD 147INSTALL 22

202

installing 21instances of structures 60INSW 147INT 147INT01 147INT1 147INT3 147integer overflow 36Intel number formats 35INTO 148INVD 148INVLPG 148IRET 148IRETD 148IRETW 148ISTRUC 60iterating over macro parameters 48Jcc 148Jcc NEAR 103JCXZ 148JECXZ 148JMP 149JMP DWORD 100jumps, mixed−size 100−k 108−l option 24label prefix 39LAHF 149LAR 149ld86 78LDMXCSR 150LDS 150LEA 150LEAVE 150LES 150LFENCE 151LFS 150LGDT 151LGS 150LIBRARY 79licence 20LIDT 151%line 63__LINE__ 58linker, free 81Linux

a.out 78as86 78ELF 75

listing file 24little−endian 35LLDT 151LMSW 151

LOADALL 151LOADALL286 151%local 62local labels 39LODSB 152LODSD 152LODSW 152logical AND 52logical OR 52logical XOR 52LOOP 152LOOPE 152LOOPNE 152LOOPNZ 152LOOPZ 152LSL 152LSS 150LTR 153−M option 24%macro 44macro library 25macro processor 41macro−local labels 46macro−params 27macros 33make 22makefile dependencies 24makefiles 21, 22Makefile.unx 22man pages 22MASKMOVDQU 153MASKMOVQ 153MASM 20MASM 28, 33, 70MAXPD 153MAXPS 153MAXSD 153MAXSS 153memory models 29, 85memory operand 32memory references 28, 33, 109MFENCE 154Microsoft OMF 70Minix 78MINPD 154MINPS 154MINSD 154MINSS 154misc subdirectory 82, 89, 95mixed−language program 84mixed−size addressing 100mixed−size instruction 100MMX registers 111

203

ModR/M byte 110, 113MODULE 79modulo operators 36MOV 155MOVAPD 155MOVAPS 156MOVD 156MOVDQA 156MOVDQ2Q 156MOVDQU 156MOVHLPS 156MOVHPD 157MOVHPS 157MOVLHPS 157MOVLPD 157MOVLPS 158MOVMSKPD 158MOVMSKPS 158MOVNTDQ 158MOVNTI 158MOVNTPD 158MOVNTPS 159MOVNTQ 159MOVQ 159MOVQ2DQ 159MOVSB 159MOVSD 159MOVSS 160MOVSW 159MOVSX 160MOVUPD 160MOVUPS 160MOVZX 160MS−DOS 69MS−DOS device drivers 84MUL 160MULPD 161MULPS 161MULSD 161MULSS 161multi−line macros 27, 44multipass optimization 26multiple section names 69multiplication 36multipush macro 48nasm.1 22NASM version 58nasm version id 58nasm version string 58__NASMDEFSEG 70nasm−devel 21nasm.exe 21nasm −f <format> −y 24

nasm −hf 24__NASM_MAJOR__ 58__NASM_MINOR__ 58nasm.out 23___NASM_PATCHLEVEL__ 58__NASM_SUBMINOR__ 58__NASM_VER__ 58__NASM_VERSION_ID__ 58nasmw.exe 21nasmXXXs.zip 21nasm−X.XX.tar.gz 21nasmXXX.zip 21ndisasm.1 22ndisasm 106ndisasm.exe 21ndisasmw.exe 21near call 29, 121near common variables 74near jump 149NEG 161NetBSD 78, 96new releases 21noalloc 76nobits 76noexec 76.nolist 50NOP 161NOT 161‘nowait’ 29nowrite 76number−overflow 27numeric constants 32, 34−o option 23, 106o16 102, 174, 180o32 102, 174, 180.OBJ 81obj 23, 70object 77, 79octal 34OF_DBG 79OF_DEFAULT 24OFFSET 28OMF 70omitted parameters 47−On option 26one’s complement 36OpenBSD 78, 96operands 31operand−size prefixes 31operating system 69

writing 100operators 36OR 161

204

ORG 69, 83, 84, 103ORPD 162orphan−labels 27, 31ORPS 162OS/2 70, 71other preprocessor directives 63OUT 162out of range, jumps 103output file format 24output formats 69OUTSB 162OUTSD 162OUTSW 162overlapping segments 37OVERLAY 71overloading

multi−line macros 45single−line macros 42

−P option 25−p option 25, 55PACKSSDW 163PACKSSWB 163PACKUSWB 163PADDB 163PADDD 163PADDQ 164PADDSB 164PADDSIW 164PADDSW 164PADDUSB 164PADDUSW 164PADDW 163PAND 164PANDN 164paradox 38PASCAL 92Pascal calling convention 90passes, assembly 38PATH 21PAUSE 165PAVEB 165PAVGB 165PAVGUSB 165PAVGW 165PCMPxx 165PDISTIB 166period 39Perl 21perverse 25PEXTRW 166PFACC 167PFADD 167PFCMPEQ 167

PFCMPGE 167PFCMPGT 167PFCMPxx 167PF2ID 167PF2IW 167, 170PFMAX 168PFMIN 168PFMUL 168PFNACC 168PFPNACC 168PFRCP 168PFRCPIT1 169PFRCPIT2 169PFRSQIT1 169PFRSQRT 169PFSUB 169PFSUBR 169PharLap 71PIC 76, 78, 96PI2FD 170PINSRW 170..plt 76PLT relocations 77, 98, 99plt relocations 99PMACHRIW 170PMADDWD 170PMAGW 171PMAXSW 171PMAXUB 171PMINSW 171PMINUB 171PMOVMSKB 171PMULHRIW 172PMULHRWA 172PMULHRWC 172PMULHUW 172PMULHW 172PMULLW 172PMULUDQ 173PMVccZB 173%pop 55POP 173POPAx 174POPFx 174POR 174position−independent code 76, 78, 96precedence 36pre−defining macros 25, 42preferred 37PREFETCH 174PREFETCHh 175PREFETCHNTA 175PREFETCHT0 175

205

PREFETCHT1 175PREFETCHT2 175pre−including files 25preprocess−only mode 26preprocessor 26, 33, 36, 41preprocessor expressions 26preprocessor loops 54preprocessor variables 43primitive directives 64PRIVATE 71proc 79, 89, 95procedure linkage table 77, 98, 99processor mode 64progbits 76program entry point 73, 81program origin 69PSADBW 175pseudo−instructions 32PSHUFD 175PSHUFHW 175PSHUFLW 176PSHUFW 176PSLLx 176PSRAx 176PSRLx 177PSUBSIW 178PSUBSxx 178PSUBUSx 178PSUBx 177PSWAPD 178PSWAPW 178PUBLIC 67, 71PUNPCKxxx 179pure binary 69%push 55PUSH 179PUSHAx 180PUSHFx 180PXOR 181quick start 28QWORD 32−r 106RCL 181RCPPS 181RCPSS 182RCR 181rdf 23, 78RDMSR 182rdoff subdirectory 22, 78, 79RDPMC 182RDSHR 182RDTSC 182redirecting errors 25

register push 180relational operators 52Relocatable Dynamic Object FileFormat 78relocations, PIC−specific 76removing contexts 55renaming contexts 56%rep 33, 54repeating 33, 54%repl 56reporting bugs 104RESB 30, 32, 38RESD 32RESQ 32REST 32restricted memory references 109RESW 32RET 182RETF 182RETN 182ROL 182ROR 182%rotate 48rotating macro parameters 48RPL 119RSDC 183RSLDT 183RSM 183RSQRTPS 183RSQRTSS 183RSTS 184−s option 25, 107SAHF 184SAL 184SALC 184SAR 184SBB 185SCASB 185SCASD 185SCASW 185searching for include files 54__SECT__ 65, 66SECTION 64

elf extensions to 76win32 extensions to 75

section alignmentin bin 70in elf 76in obj 71in win32 75

section, bin extensions to 69SEG 36, 37, 71

206

SEGMENT 64elf extensions to 71

segment address 36, 37segment alignment

in bin 70in obj 71

segment names, Borland Pascal 91segment override 29, 31segment registers 111segments 37

groups of 72separator character 28SETcc 185SFENCE 186SGDT 186shared libraries 78, 96shared library 77shift command 48SHL 186SHLD 187SHR 186SHRD 187SHUFPD 187SHUFPS 188SIB byte 110, 113SIDT 186signed division 36signed modulo 36single−line macros 41size, of symbols 77SLDT 186SMI 188SMINT 188SMINTOLD 188SMSW 188Solaris x86 75−soname 99sound 32source code 21source−listing file 24SQRTPD 188SQRTPS 188SQRTSD 188SQRTSS 189square brackets 28, 33sse condition predicates 111STACK 71stack frame 133%stacksize 62standard macros 58standardised section names 65, 75, 76, 78,

79..start 73, 81

status flags 112STC 189STD 189stderr 24stdout 25STI 189STMXCSR 189STOSB 189STOSD 189STOSW 189STR 189STRICT 37string constant 32string handling in macros 44string length 44%strlen 44STRUC 59, 66, 88, 95stub preprocessor 26SUB 190SUBPD 190SUBPS 190SUBSD 190SUBSS 190%substr 44sub−strings 44subtraction 36suppressible warning 27suppressing preprocessing 26SVDC 191SVLDT 191SVTS 191switching between sections 64..sym 76symbol sizes, specifying 77symbol types, specifying 77symbols

exporting from DLLs 73importing from DLLs 72

synchronisation 107.SYS 69, 84SYSCALL 191SYSENTER 191SYSEXIT 192SYSRET 192−t 27TASM 20, 27tasm 28, 70tasm compatible preprocessordirectives 61TBYTE 29TEST 193test subdirectory 81test registers 111

207

testingarbitrary numeric expressions 52context stack 51exact text identity 52multi−line macro existence 51single−line macro existence 51token types 52

.text 76, 78, 79_TEXT 86TIMES 32, 33, 38, 103, 104TLINK 84trailing colon 31two−pass assembler 38TWORD 29, 32type, of symbols 77−U option 26−u option 26, 106UCOMISD 193UCOMISS 193UD0 193UD1 193UD2 193UMOV 194unary operators 36%undef 26, 43undefining macros 26underscore, in C symbols 84uninitialised 32uninitialised storage 29Unix 21

SCO 75source archive 21System V 75

UnixWare 75UNPCKHPD 194UNPCKHPS 194UNPCKLPD 194UNPCKLPS 194unrolled loops 33unsigned division 36unsigned modulo 36UPPERCASE 28, 72USE16 64, 71USE32 64, 71user−defined errors 53user−level assembler directives 58user−level directives 64−v option 27VAL 81valid characters 31variable types 29VERR 195version 27

version number of NASM 58VERW 195Visual C++ 74−w option 27WAIT 195warnings 27WBINVD 195Win32 21, 23, 70, 74, 93Windows 81Windows 95 21Windows NT 21write 76writing operating systems 100WRMSR 195WRSHR 195WRT 37, 71, 76, 78WRT ..got 98WRT ..gotoff 97WRT ..gotpc 97WRT ..plt 99WRT ..sym 99WWW page 20www.cpan.org 21www.delorie.com 81www.pcorner.com 81XADD 195XBTS 196XCHG 196%xdefine 42x2ftp.oulu.fi 81%xidefine 42XLATB 196XOR 196XORPD 197XORPS 197

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nasm — the netwide assembler · 2002-09-03 · 7.5 interfacing to borland pascal programs . . . ....

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